diff --git a/crates/ui-egui/src/combine_grid.rs b/crates/ui-egui/src/combine_grid.rs
index 7fadf78cb..e0336d969 100644
--- a/crates/ui-egui/src/combine_grid.rs
+++ b/crates/ui-egui/src/combine_grid.rs
@@ -557,7 +557,14 @@ fn file_gap(cells: &[Cell], at: usize, files: usize) -> usize {
 }
 
 /// The grid of files, in the card that holds the list. Returns what the user did.
-pub(crate) fn grid(app: &mut PdfCraftApp, ui: &mut egui::Ui, t: &Tokens, checks: &[Result<usize, String>], selected: &[bool]) -> Option<RowAction> {
+pub(crate) fn grid(
+    app: &mut PdfCraftApp,
+    ui: &mut egui::Ui,
+    t: &Tokens,
+    checks: &[Result<usize, String>],
+    selected: &[bool],
+    twice: &[bool],
+) -> Option<RowAction> {
     let ctx = ui.ctx().clone();
     let ppp = ctx.pixels_per_point();
     let mut action = None;
@@ -566,13 +573,6 @@ pub(crate) fn grid(app: &mut PdfCraftApp, ui: &mut egui::Ui, t: &Tokens, checks:
     // The card dragged may scroll out of the rows drawn, so its release is watched here.
     let released = drag.is_some() && ctx.input(|i| i.pointer.any_released());
     let n = app.combine_draft.len();
-    // The same file twice is allowed (e.g. a cover sheet), but probably a mistake.
-    // (A file's parts are the one file, not copies: counted by file.)
-    let mut copies: HashMap<(&str, usize), std::collections::BTreeSet<u64>> = HashMap::new();
-    for f in &app.combine_draft {
-        copies.entry((f.name.as_str(), f.bytes.len())).or_default().insert(f.group);
-    }
-    let twice: Vec<bool> = app.combine_draft.iter().map(|f| copies.get(&(f.name.as_str(), f.bytes.len())).is_some_and(|g| g.len() > 1)).collect();
     let places = layout(app);
     // Each file's first place (for the keys and to scroll to it).
     let mut first_place = vec![0usize; n];
diff --git a/crates/ui-egui/src/combine_ui.rs b/crates/ui-egui/src/combine_ui.rs
index f7dc21835..2d2bb173a 100644
--- a/crates/ui-egui/src/combine_ui.rs
+++ b/crates/ui-egui/src/combine_ui.rs
@@ -180,6 +180,8 @@ pub struct CombineTab {
     unlock: Option<UnlockPrompt>,
     /// Bumped on every change to the list, so a grid drag that began before one is dropped.
     pub(crate) revision: u64,
+    /// Exact-content duplicate warnings, shared by both views until the list changes.
+    duplicates: Option<(u64, Arc<[bool]>)>,
     /// The file the keyboard moved to, for the grid to scroll into view.
     pub(crate) reveal: Option<u64>,
     /// Files per grid row as last drawn (Up/Down move a row in the grid).
@@ -423,6 +425,49 @@ pub struct CombineFile {
     checked: Option<(String, RangeCheck)>,
 }
 
+impl CombineTab {
+    fn duplicate_warnings(&mut self, files: &[CombineFile]) -> Arc<[bool]> {
+        if let Some((revision, flags)) = &self.duplicates
+            && *revision == self.revision
+        {
+            return flags.clone();
+        }
+        // Only same-named, same-sized files can be repeats. Avoid reading the bytes of
+        // unrelated files, and never count a source split into parts as several additions.
+        let mut candidates: HashMap<(&str, usize), Vec<usize>> = HashMap::new();
+        for (i, f) in files.iter().enumerate() {
+            candidates.entry((&f.name, f.bytes.len())).or_default().push(i);
+        }
+        let mut twice = vec![false; files.len()];
+        for indices in candidates.values() {
+            let Some(first) = indices.first().and_then(|i| files.get(*i)) else { continue };
+            if indices.iter().all(|i| files.get(*i).is_some_and(|f| f.group == first.group)) {
+                continue;
+            }
+            // Slice keys check exact bytes even when their hashes collide.
+            let mut contents: HashMap<&[u8], Vec<usize>> = HashMap::new();
+            for &i in indices {
+                if let Some(f) = files.get(i) {
+                    contents.entry(f.bytes.as_slice()).or_default().push(i);
+                }
+            }
+            for copies in contents.values() {
+                let Some(first) = copies.first().and_then(|i| files.get(*i)) else { continue };
+                if copies.iter().any(|i| files.get(*i).is_some_and(|f| f.group != first.group)) {
+                    for &i in copies {
+                        if let Some(flag) = twice.get_mut(i) {
+                            *flag = true;
+                        }
+                    }
+                }
+            }
+        }
+        let flags: Arc<[bool]> = twice.into();
+        self.duplicates = Some((self.revision, flags.clone()));
+        flags
+    }
+}
+
 impl CombineFile {
     fn take(&mut self, a: Assessed, password: Option<Secret>) {
         self.pages = a.pages;
@@ -591,6 +636,7 @@ pub(crate) fn page(app: &mut PdfCraftApp, ui: &mut egui::Ui) {
     // list, so the Warnings column can say so before anything is combined.
     app.combine_conflicts.refresh(&app.session, &app.combine_draft);
     let conflicts = app.combine_conflicts.per_file.clone();
+    let twice = app.combine_tab.duplicate_warnings(&app.combine_draft);
     let n = app.combine_draft.len();
     let ids: Vec<u64> = app.combine_draft.iter().map(|f| f.id).collect();
     app.combine_tab.selected.retain(|id| ids.contains(id));
@@ -778,11 +824,11 @@ pub(crate) fn page(app: &mut PdfCraftApp, ui: &mut egui::Ui) {
                 // The view as the toolbar left it: a click on List view this frame starts no
                 // thumbnail renders.
                 } else if app.combine_view == CombineView::Grid {
-                    if let Some(action) = crate::combine_grid::grid(app, ui, &t, &checks, &selected) {
+                    if let Some(action) = crate::combine_grid::grid(app, ui, &t, &checks, &selected, &twice) {
                         events.action = Some(action);
                     }
                 } else {
-                    let table_events = table(app, ui, &t, &checks, &selected, &conflicts);
+                    let table_events = table(app, ui, &t, &checks, &selected, &conflicts, &twice);
                     events.range_focused |= table_events.range_focused;
                     events.range_changed |= table_events.range_changed;
                     if table_events.action.is_some() {
@@ -1226,15 +1272,9 @@ fn table(
     checks: &[Result<usize, String>],
     selected: &[bool],
     conflicts: &[Vec<(String, String)>],
+    twice: &[bool],
 ) -> TableEvents {
     let mut events = TableEvents::default();
-    // The same file twice is allowed (e.g. a cover sheet), but probably a mistake.
-    // (A file's parts are the one file, not copies.)
-    let twice: Vec<bool> = app
-        .combine_draft
-        .iter()
-        .map(|f| app.combine_draft.iter().filter(|g| g.group != f.group && g.name == f.name && g.bytes.len() == f.bytes.len()).count() > 0)
-        .collect();
     let dragging = egui::DragAndDrop::has_payload_of_type::<usize>(ui.ctx());
     // Until the user resizes a column, the widths follow the window.
     let gap = ui.spacing().item_spacing.x;
@@ -1244,7 +1284,7 @@ fn table(
         app.combine_tab.layout = app.combine_tab.layout.wrapping_add(1);
     }
     let widths = app.combine_columns.resolved(available);
-    let rows = Rows { checks, selected, twice: &twice, conflicts, dragging, sort: app.combine_tab.sort };
+    let rows = Rows { checks, selected, twice, conflicts, dragging, sort: app.combine_tab.sort };
     let mut heading_event = None;
     let mut shown = None;
     egui::ScrollArea::horizontal()
@@ -2573,6 +2613,29 @@ mod tests {
         pdfcraft_cos::write_full(&doc, &Default::default()).unwrap()
     }
 
+    #[test]
+    fn duplicate_warnings_reuse_results_and_track_removal_and_parts() {
+        let bytes = locked();
+        let mut app = PdfCraftApp::new();
+        app.stage_combine(vec![("same.pdf".into(), bytes.clone())]);
+        let mut part = app.combine_draft[0].clone();
+        part.id = 99;
+        app.combine_draft.push(part);
+        let first = app.combine_tab.duplicate_warnings(&app.combine_draft);
+        assert_eq!(&*first, &[false, false], "parts of one source are not duplicate additions");
+        let again = app.combine_tab.duplicate_warnings(&app.combine_draft);
+        assert!(Arc::ptr_eq(&first, &again), "unchanged frames reuse the result without scanning files");
+        app.stage_combine(vec![("same.pdf".into(), bytes)]);
+        assert_eq!(&*app.combine_tab.duplicate_warnings(&app.combine_draft), &[true, true, true]);
+        let duplicate = app.combine_draft.last().unwrap().id;
+        app.combine_apply(RowAction::RemoveFile(duplicate));
+        assert_eq!(&*app.combine_tab.duplicate_warnings(&app.combine_draft), &[false, false]);
+        app.combine_history_step(true);
+        assert_eq!(&*app.combine_tab.duplicate_warnings(&app.combine_draft), &[true, true, true]);
+        app.close_combine_tab();
+        assert!(app.combine_tab.duplicate_warnings(&app.combine_draft).is_empty());
+    }
+
     #[test]
     fn every_read_of_a_file_is_new_even_after_undo() {
         // Regression: the read counter lived in the undo history, so unlock → undo → unlock (with
diff --git a/crates/ui-egui/tests/combine_grid.rs b/crates/ui-egui/tests/combine_grid.rs
index 8e7ab6bc4..e0d2668b8 100644
--- a/crates/ui-egui/tests/combine_grid.rs
+++ b/crates/ui-egui/tests/combine_grid.rs
@@ -126,6 +126,44 @@ fn names(h: &Harness<'static, PdfCraftApp>) -> Vec<String> {
     h.state().combine_draft.iter().map(|f| f.name.clone()).collect()
 }
 
+fn check_duplicate_warnings(view: &str) {
+    let first = fixture_sized(1, 200.0, 300.0);
+    let other = fixture_sized(1, 300.0, 200.0);
+    assert_eq!(first.len(), other.len());
+    assert_ne!(first, other);
+    let mut h = grid_of(vec![("report.pdf", first.clone()), ("report.pdf", other)], vec2(1400.0, 900.0));
+    h.state_mut().set_option("combine-view", view).unwrap();
+    h.run_steps(2);
+    let warning_count = |h: &mut Harness<'static, PdfCraftApp>| {
+        if view == "grid" {
+            let at = h.get_all_by_label("report.pdf").next().unwrap().rect().center();
+            h.hover_at(at);
+            h.run_steps(60);
+        }
+        h.query_all_by_label_contains("Added more than once").count()
+    };
+    assert_eq!(warning_count(&mut h), 0, "different equal-sized PDFs are not duplicates in {view}");
+    h.state_mut().use_files(FilePurpose::Combine, vec![("report.pdf".into(), first)]);
+    h.run_steps(3);
+    assert_eq!(warning_count(&mut h), if view == "grid" { 1 } else { 2 }, "only the two identical files are warned");
+    h.key_press_modifiers(Modifiers::COMMAND, Key::Z);
+    h.run_steps(3);
+    assert_eq!(warning_count(&mut h), 0, "undo removes the duplicate warning");
+    h.key_press_modifiers(Modifiers::COMMAND | Modifiers::SHIFT, Key::Z);
+    h.run_steps(3);
+    assert_eq!(warning_count(&mut h), if view == "grid" { 1 } else { 2 }, "redo restores the duplicate warning");
+}
+
+#[test]
+fn list_duplicate_warnings_compare_contents_and_follow_undo() {
+    check_duplicate_warnings("list");
+}
+
+#[test]
+fn grid_duplicate_warnings_compare_contents_and_follow_undo() {
+    check_duplicate_warnings("grid");
+}
+
 /// Run frames until `done`, giving render threads time; whether it happened.
 fn settle(h: &mut Harness<'static, PdfCraftApp>, done: impl Fn(&PdfCraftApp) -> bool) -> bool {
     for _ in 0..400 {