diff --git a/src/web.rs b/src/web.rs
index 08ef2a7..6b75594 100644
--- a/src/web.rs
+++ b/src/web.rs
@@ -24,7 +24,7 @@
 use std::collections::{HashMap, HashSet};
 use std::path::{Path as StdPath, PathBuf};
 use std::process::Stdio;
-use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
+use std::sync::atomic::{AtomicBool, AtomicU64, AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex};
 
 use axum::{
@@ -133,6 +133,43 @@ pub struct WebState {
     /// re-serializing. The Arc lets concurrent responses share one
     /// allocation. Cleared / replaced lazily on the next miss.
     pub library_body_cache: Mutex<Option<(u64, std::sync::Arc<String>)>>,
+    /// Background perceptual-dedup job state. The expensive fingerprint pass
+    /// runs on its own thread; this carries live progress + the last result
+    /// so `/api/maintenance/dedup/status` can be polled. `Arc` so the worker
+    /// thread can hold a handle past the request that started it.
+    pub dedup: std::sync::Arc<DedupState>,
+}
+
+/// Live state for the background perceptual-dedup job (see
+/// [`post_dedup_scan`]). One job runs at a time, guarded by `running`.
+#[derive(Default)]
+pub struct DedupState {
+    running: AtomicBool,
+    done: AtomicUsize,
+    total: AtomicUsize,
+    result: Mutex<Option<Vec<SimilarGroup>>>,
+    error: Mutex<Option<String>>,
+}
+
+/// One video in a perceptual-similarity group, with everything the review UI
+/// needs to show it and (optionally) delete its files.
+#[derive(serde::Serialize, Clone)]
+pub struct SimilarVideo {
+    video_id: String,
+    title: String,
+    channel: String,
+    location: String,
+    file_size: Option<u64>,
+    /// Absolute paths of the video + its sidecars, for the delete action.
+    files: Vec<String>,
+    /// The copy the UI recommends keeping (largest file in the group).
+    recommended_keep: bool,
+}
+
+/// A cluster of videos that share visual content across different IDs.
+#[derive(serde::Serialize, Clone)]
+pub struct SimilarGroup {
+    videos: Vec<SimilarVideo>,
 }
 
 /// Failed-login tracking entry. After [`LOGIN_LOCKOUT_AFTER`] failures from
@@ -1976,6 +2013,132 @@ struct RemoveRequest {
     paths: Vec<PathBuf>,
 }
 
+/// `POST /api/maintenance/dedup/scan` — start (or no-op if already running)
+/// the background perceptual-dedup pass. Fingerprints any new/changed videos
+/// (cached by mtime), then groups by visual similarity. Returns immediately;
+/// poll `/api/maintenance/dedup/status` for progress + results.
+async fn post_dedup_scan(State(state): State<Arc<WebState>>) -> impl IntoResponse {
+    if state.dedup.running.swap(true, Ordering::SeqCst) {
+        return Json(serde_json::json!({ "started": false, "running": true }));
+    }
+    state.dedup.done.store(0, Ordering::Relaxed);
+    state.dedup.total.store(0, Ordering::Relaxed);
+    *state.dedup.error.lock_recover() = None;
+
+    // Snapshot what the worker needs from the library (cheap clones of paths +
+    // display strings) so it doesn't hold the lock during the ffmpeg pass.
+    let (inputs, by_path, valid_paths) = {
+        let lib = state.library.lock_recover();
+        let root = state.library_root.clone();
+        let mut inputs: Vec<crate::fingerprint::FpInput> = Vec::new();
+        let mut by_path: HashMap<String, SimilarVideo> = HashMap::new();
+        let mut valid_paths: HashSet<String> = HashSet::new();
+        for ch in lib.iter() {
+            let channel = ch.name.clone();
+            let videos = ch.videos.iter().chain(ch.playlists.iter().flat_map(|p| p.videos.iter()));
+            for v in videos {
+                let Some(vp) = &v.video_path else { continue }; // need a real file
+                let path_str = vp.display().to_string();
+                valid_paths.insert(path_str.clone());
+                inputs.push(crate::fingerprint::FpInput {
+                    path: vp.clone(),
+                    mtime_unix: v.mtime_unix.map(|m| m as i64).unwrap_or(0),
+                    video_id: v.id.clone(),
+                    duration_secs: v.duration_secs.unwrap_or(0.0),
+                });
+                let mut files = vec![path_str.clone()];
+                for p in [v.thumb_path.as_ref(), v.info_path.as_ref(), v.description_path.as_ref()]
+                    .into_iter().flatten() { files.push(p.display().to_string()); }
+                for s in &v.subtitles { files.push(s.path.display().to_string()); }
+                let location = vp.parent()
+                    .map(|d| d.strip_prefix(&root).unwrap_or(d).display().to_string())
+                    .unwrap_or_default();
+                by_path.insert(path_str, SimilarVideo {
+                    video_id: v.id.clone(), title: v.title.clone(), channel: channel.clone(),
+                    location, file_size: v.file_size, files, recommended_keep: false,
+                });
+            }
+        }
+        (inputs, by_path, valid_paths)
+    };
+
+    let db = state.db.clone();
+    let dedup = state.dedup.clone();
+    std::thread::spawn(move || run_dedup(db, dedup, inputs, by_path, valid_paths));
+    Json(serde_json::json!({ "started": true }))
+}
+
+/// `GET /api/maintenance/dedup/status` — progress + results of the dedup job.
+async fn get_dedup_status(State(state): State<Arc<WebState>>) -> impl IntoResponse {
+    let d = &state.dedup;
+    Json(serde_json::json!({
+        "running": d.running.load(Ordering::Relaxed),
+        "done":    d.done.load(Ordering::Relaxed),
+        "total":   d.total.load(Ordering::Relaxed),
+        "groups":  d.result.lock_recover().clone(),
+        "error":   d.error.lock_recover().clone(),
+    }))
+}
+
+/// Worker body for the dedup job: mtime-gate, fingerprint the missing videos
+/// in parallel, prune vanished entries, then group by visual similarity and
+/// stash the result. Always clears `running` on exit.
+fn run_dedup(
+    db: Database,
+    dedup: std::sync::Arc<DedupState>,
+    inputs: Vec<crate::fingerprint::FpInput>,
+    by_path: HashMap<String, SimilarVideo>,
+    valid_paths: HashSet<String>,
+) {
+    use crate::fingerprint;
+    let outcome: Result<Vec<SimilarGroup>, String> = (|| {
+        let known = db.fingerprint_mtimes().map_err(|e| e.to_string())?;
+        let todo: Vec<fingerprint::FpInput> = inputs
+            .into_iter()
+            .filter(|i| known.get(&i.path.display().to_string()) != Some(&i.mtime_unix))
+            .collect();
+        dedup.total.store(todo.len(), Ordering::Relaxed);
+
+        let workers = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(4);
+        let computed = fingerprint::compute_batch(todo, workers, &dedup.done);
+        for c in &computed {
+            let _ = db.upsert_fingerprint(
+                &c.input.path.display().to_string(), c.input.mtime_unix,
+                &c.input.video_id, c.input.duration_secs, &c.hashes,
+            );
+        }
+        let _ = db.prune_fingerprints(&valid_paths);
+
+        let stored = db.load_fingerprints().map_err(|e| e.to_string())?;
+        let records: Vec<fingerprint::FpRecord> = stored.iter().map(|s| fingerprint::FpRecord {
+            video_id: s.video_id.clone(), duration_secs: s.duration_secs, hashes: s.hashes.clone(),
+        }).collect();
+        let groups_idx = fingerprint::group_similar(
+            &records, fingerprint::DEFAULT_DUR_TOL,
+            fingerprint::DEFAULT_MAX_HAM, fingerprint::DEFAULT_MIN_MATCH,
+        );
+
+        let mut groups = Vec::new();
+        for g in groups_idx {
+            let mut vids: Vec<SimilarVideo> =
+                g.iter().filter_map(|&i| by_path.get(&stored[i].path).cloned()).collect();
+            if vids.len() < 2 { continue; } // stale paths dropped out
+            // Recommend keeping the largest file (best quality) in each group.
+            let keep = vids.iter().enumerate()
+                .max_by_key(|(_, v)| v.file_size.unwrap_or(0)).map(|(i, _)| i);
+            for (i, v) in vids.iter_mut().enumerate() { v.recommended_keep = Some(i) == keep; }
+            groups.push(SimilarGroup { videos: vids });
+        }
+        Ok(groups)
+    })();
+
+    match outcome {
+        Ok(groups) => *dedup.result.lock_recover() = Some(groups),
+        Err(e) => *dedup.error.lock_recover() = Some(e),
+    }
+    dedup.running.store(false, Ordering::SeqCst);
+}
+
 /// `POST /api/maintenance/remove` — delete the listed duplicate files.
 /// Paths outside the library root are refused.
 async fn post_maintenance_remove(
@@ -2611,6 +2774,7 @@ async fn serve(config: Config, shutdown_rx: std::sync::mpsc::Receiver<()>) {
         progress_tx,
         login_attempts: Mutex::new(HashMap::new()),
         library_body_cache: Mutex::new(None),
+        dedup: std::sync::Arc::new(DedupState::default()),
     });
 
     // Broadcast progress snapshots to WebSocket subscribers. Ticks fast
@@ -2721,6 +2885,8 @@ async fn serve(config: Config, shutdown_rx: std::sync::mpsc::Receiver<()>) {
         .route("/api/plex/generate", post(post_plex_generate))
         .route("/api/maintenance/scan", get(get_maintenance_scan))
         .route("/api/maintenance/remove", post(post_maintenance_remove))
+        .route("/api/maintenance/dedup/scan", post(post_dedup_scan))
+        .route("/api/maintenance/dedup/status", get(get_dedup_status))
         .route("/api/maintenance/repair/:id", post(post_maintenance_repair))
         .route("/api/cookies", get(get_cookies).post(post_cookies).delete(delete_cookies))
         .route("/api/scheduler/run", post(post_scheduler_run))
diff --git a/src/web_ui/index.html b/src/web_ui/index.html
index 48628e1..c981b06 100644
--- a/src/web_ui/index.html
+++ b/src/web_ui/index.html
@@ -2162,7 +2162,86 @@ function renderMaintenance(r){
       </div>`;
     }
   }
+  h+=`<h3 style="font-size:13px;margin:16px 0 8px">Similar content (perceptual)</h3>
+    <div class="settings-hint" style="margin-bottom:8px">Finds the same video re-uploaded under a different ID — mirrors, re-encodes, resolution changes — by comparing sampled frames. The first scan fingerprints your library (a few minutes); after that it's cached, so re-scans are quick.</div>
+    <div id="dedup-area"></div>`;
   body.innerHTML=h;
+  initDedupArea();
+}
+
+/* ── Perceptual dedup (similar content) ─────────────────────────── */
+let dedupTimer=null;
+async function initDedupArea(){
+  if(dedupTimer){clearInterval(dedupTimer);dedupTimer=null}
+  try{
+    const s=await(await api('/api/maintenance/dedup/status')).json();
+    if(s.running){renderDedupProgress(s);startDedupPoll();}
+    else if(s.groups){renderSimilarGroups(s.groups);}
+    else dedupIdle(s.error);
+  }catch(e){const a=document.getElementById('dedup-area');if(a)a.innerHTML=`<div style="color:#f87171;font-size:12px">${esc(e.message)}</div>`}
+}
+function dedupIdle(err){
+  const a=document.getElementById('dedup-area');if(!a)return;
+  a.innerHTML=`${err?`<div style="color:#f87171;font-size:12px;margin-bottom:6px">Last scan error: ${esc(err)}</div>`:''}
+    <button class="primary" onclick="startDedup(this)">🔍 Scan for similar content</button>`;
+}
+async function startDedup(btn){
+  if(btn)btn.disabled=true;
+  try{await api('/api/maintenance/dedup/scan',{method:'POST'});startDedupPoll();}
+  catch(e){setStatus('Error: '+e.message);if(btn)btn.disabled=false}
+}
+function startDedupPoll(){
+  if(dedupTimer)clearInterval(dedupTimer);
+  dedupTimer=setInterval(async()=>{
+    if(!document.getElementById('dedup-area')){clearInterval(dedupTimer);dedupTimer=null;return} // modal closed
+    let s;try{s=await(await api('/api/maintenance/dedup/status')).json()}catch{return}
+    if(s.running){renderDedupProgress(s);return}
+    clearInterval(dedupTimer);dedupTimer=null;
+    if(s.error)dedupIdle(s.error); else renderSimilarGroups(s.groups||[]);
+  },800);
+}
+function renderDedupProgress(s){
+  const a=document.getElementById('dedup-area');if(!a)return;
+  const pct=s.total?Math.round(100*s.done/s.total):0;
+  a.innerHTML=`<div style="font-size:12px;color:var(--muted);margin-bottom:6px">Fingerprinting ${s.done} / ${s.total||'…'} new videos…</div>
+    <div style="height:6px;background:var(--bg);border-radius:3px;overflow:hidden"><div style="height:100%;width:${pct}%;background:var(--accent);transition:width .3s"></div></div>`;
+}
+function renderSimilarGroups(groups){
+  const a=document.getElementById('dedup-area');if(!a)return;
+  let h=`<div style="display:flex;gap:8px;align-items:center;margin-bottom:8px">
+    <span style="font-size:12px;color:var(--muted)">${groups.length} similar group${groups.length===1?'':'s'}</span>
+    <button onclick="startDedup(this)">⟳ Re-scan</button></div>`;
+  if(!groups.length){h+='<div style="color:var(--muted);font-size:12px">No visually-similar duplicates found across different IDs. 🎉</div>';a.innerHTML=h;return}
+  groups.forEach((g,gi)=>{
+    h+=`<div style="border:1px solid var(--border);border-radius:6px;padding:8px;margin-bottom:8px">
+      <div style="font-weight:600;font-size:12px;margin-bottom:6px">Group ${gi+1} — ${g.videos.length} copies</div>`;
+    g.videos.forEach(v=>{
+      const tag=v.recommended_keep?'<span style="color:#4ade80">keep</span>':'<span style="color:#f87171">remove</span>';
+      const size=v.file_size?fmtSize(v.file_size):'no video';
+      h+=`<label style="display:flex;align-items:center;gap:8px;font-size:12px;padding:3px 0">
+        <input type="checkbox" class="sim-chk" data-files='${esc(JSON.stringify(v.files))}' ${v.recommended_keep?'':'checked'}>
+        <span style="flex:1;overflow:hidden;text-overflow:ellipsis;white-space:nowrap" title="${esc(v.location)}">${esc(v.title)} <span style="color:var(--muted)">· ${esc(v.channel)} · ${size} · [${esc(v.video_id)}]</span></span>
+        ${tag}
+      </label>`;
+    });
+    h+=`</div>`;
+  });
+  h+=`<button class="primary" onclick="removeSimilar(this)">🗑 Remove checked copies</button>`;
+  a.innerHTML=h;
+}
+async function removeSimilar(btn){
+  const chks=[...document.querySelectorAll('.sim-chk:checked')];
+  let paths=[];
+  for(const c of chks){try{paths=paths.concat(JSON.parse(c.dataset.files))}catch{}}
+  if(!paths.length){setStatus('Nothing selected.');return}
+  if(!confirm(`Delete ${paths.length} file(s) across the checked copies? This cannot be undone.`))return;
+  btn.disabled=true;
+  try{
+    const r=await(await api('/api/maintenance/remove',{method:'POST',headers:{'Content-Type':'application/json'},body:JSON.stringify({paths})})).json();
+    setStatus(`Removed ${r.removed} file(s)`+(r.errors&&r.errors.length?`, ${r.errors.length} error(s)`:''));
+    await loadLibrary();
+    startDedup(); // re-scan (fingerprints cached) so deleted copies drop out
+  }catch(e){setStatus('Error: '+e.message);btn.disabled=false}
 }
 
 async function removeDuplicates(btn){
diff --git a/tests/api.rs b/tests/api.rs
index dc32471..c202b6a 100644
--- a/tests/api.rs
+++ b/tests/api.rs
@@ -25,6 +25,13 @@ fn have_curl() -> bool {
         .status().map(|s| s.success()).unwrap_or(false)
 }
 
+/// True if `ffmpeg` is usable — the perceptual-dedup test needs it to both
+/// generate fixtures and fingerprint them; it skips otherwise.
+fn have_ffmpeg() -> bool {
+    Command::new("ffmpeg").arg("-version").stdout(Stdio::null()).stderr(Stdio::null())
+        .status().map(|s| s.success()).unwrap_or(false)
+}
+
 /// A running `yt-offline --web` child against a scratch dir. Killed and
 /// its dir removed on drop.
 struct Server {
@@ -351,3 +358,43 @@ fn full_text_search_indexes_titles_and_descriptions() {
     assert!(!s.get("/api/search?q=quantumchromodynamics").1.contains("vid123"),
             "unrelated query must not hit");
 }
+
+#[test]
+fn perceptual_dedup_groups_reencodes() {
+    if !have_curl() { eprintln!("skip: no curl"); return; }
+    if !have_ffmpeg() { eprintln!("skip: no ffmpeg"); return; }
+    let s = Server::start();
+    let chan = s.dir.join("ch/channels/Demo");
+    std::fs::create_dir_all(&chan).unwrap();
+
+    let gen = |args: &[&str]| {
+        let ok = Command::new("ffmpeg").arg("-nostdin").arg("-y").arg("-loglevel").arg("error")
+            .args(args).status().map(|st| st.success()).unwrap_or(false);
+        assert!(ok, "ffmpeg gen failed: {args:?}");
+    };
+    let orig = chan.join("orig [aaa].mp4");
+    let reenc = chan.join("reenc [bbb].mp4");
+    let diff = chan.join("diff [ccc].mp4");
+    gen(&["-f","lavfi","-i","testsrc=duration=20:size=640x480:rate=10", orig.to_str().unwrap()]);
+    gen(&["-i", orig.to_str().unwrap(), "-vf","scale=320:240","-r","15","-c:v","libx264","-crf","38", reenc.to_str().unwrap()]);
+    gen(&["-f","lavfi","-i","testsrc2=duration=20:size=640x480:rate=10", diff.to_str().unwrap()]);
+    for stem in ["orig [aaa]", "reenc [bbb]", "diff [ccc]"] {
+        std::fs::write(chan.join(format!("{stem}.info.json")), r#"{"duration":20.0}"#).unwrap();
+    }
+
+    assert_eq!(s.post("/api/rescan", "").0, 200);
+    assert_eq!(s.post("/api/maintenance/dedup/scan", "").0, 200);
+
+    // Poll until the background job finishes (fingerprinting 3 short clips).
+    let mut body = String::new();
+    for _ in 0..160 {
+        let (_, b) = s.get("/api/maintenance/dedup/status");
+        if field(&b, "running") == Some("false") { body = b; break; }
+        std::thread::sleep(std::time::Duration::from_millis(250));
+    }
+    assert!(field(&body, "running") == Some("false"), "dedup job never finished: {body}");
+    // orig + its re-encode group together; the unrelated clip stays out.
+    assert!(body.contains("aaa"), "group should contain orig: {body}");
+    assert!(body.contains("bbb"), "group should contain the re-encode: {body}");
+    assert!(!body.contains("ccc"), "unrelated video must not be grouped: {body}");
+}