diff --git a/crates/ui-egui/locales/de.json b/crates/ui-egui/locales/de.json
index 64c70e031..eb237a2e8 100644
--- a/crates/ui-egui/locales/de.json
+++ b/crates/ui-egui/locales/de.json
@@ -1914,5 +1914,7 @@
   "Failed": "Fehlgeschlagen",
   "Stopped": "Angehalten",
   "Stopped by an error (see the log)": "Durch einen Fehler angehalten (siehe Protokoll)",
-  "Not saved": "Nicht gespeichert"
+  "Not saved": "Nicht gespeichert",
+  "A folder is being synchronized": "Ein Ordner wird gerade synchronisiert",
+  "Wait for the import to finish": "Warten Sie, bis der Import abgeschlossen ist"
 }
diff --git a/crates/ui-egui/locales/es.json b/crates/ui-egui/locales/es.json
index d023cf6c4..c40f6372c 100644
--- a/crates/ui-egui/locales/es.json
+++ b/crates/ui-egui/locales/es.json
@@ -1759,5 +1759,7 @@
   "Failed": "Con errores",
   "Stopped": "Detenido",
   "Stopped by an error (see the log)": "Detenido por un error (ver el registro)",
-  "Not saved": "Sin guardar"
+  "Not saved": "Sin guardar",
+  "A folder is being synchronized": "Se está sincronizando una carpeta",
+  "Wait for the import to finish": "Espera a que termine la importación"
 }
diff --git a/crates/ui-egui/locales/ja.json b/crates/ui-egui/locales/ja.json
index 9c5072025..bd55af76c 100644
--- a/crates/ui-egui/locales/ja.json
+++ b/crates/ui-egui/locales/ja.json
@@ -1759,5 +1759,7 @@
   "Failed": "失敗",
   "Stopped": "停止しました",
   "Stopped by an error (see the log)": "エラーで停止しました（ログを参照）",
-  "Not saved": "保存されていません"
+  "Not saved": "保存されていません",
+  "A folder is being synchronized": "フォルダを同期中です",
+  "Wait for the import to finish": "読み込みが終わるまでお待ちください"
 }
diff --git a/crates/ui-egui/locales/pt-br.json b/crates/ui-egui/locales/pt-br.json
index 21a3d0c84..7e9511241 100644
--- a/crates/ui-egui/locales/pt-br.json
+++ b/crates/ui-egui/locales/pt-br.json
@@ -1759,5 +1759,7 @@
   "Failed": "Com falha",
   "Stopped": "Interrompido",
   "Stopped by an error (see the log)": "Interrompido por um erro (veja o log)",
-  "Not saved": "Não salvo"
+  "Not saved": "Não salvo",
+  "A folder is being synchronized": "Uma pasta está sendo sincronizada",
+  "Wait for the import to finish": "Aguarde a importação terminar"
 }
diff --git a/crates/ui-egui/locales/ru.json b/crates/ui-egui/locales/ru.json
index 952ef5c92..4f3689f7d 100644
--- a/crates/ui-egui/locales/ru.json
+++ b/crates/ui-egui/locales/ru.json
@@ -1433,5 +1433,7 @@
   "Failed": "Ошибки",
   "Stopped": "Остановлено",
   "Stopped by an error (see the log)": "Остановлено из-за ошибки (см. журнал)",
-  "Not saved": "Не сохранено"
+  "Not saved": "Не сохранено",
+  "A folder is being synchronized": "Идёт синхронизация папки",
+  "Wait for the import to finish": "Дождитесь окончания импорта"
 }
diff --git a/crates/ui-egui/locales/zh-hans.json b/crates/ui-egui/locales/zh-hans.json
index a67df15a8..f12273d05 100644
--- a/crates/ui-egui/locales/zh-hans.json
+++ b/crates/ui-egui/locales/zh-hans.json
@@ -1759,5 +1759,7 @@
   "Failed": "失败",
   "Stopped": "已停止",
   "Stopped by an error (see the log)": "因错误而停止（见日志）",
-  "Not saved": "未保存"
+  "Not saved": "未保存",
+  "A folder is being synchronized": "正在同步文件夹",
+  "Wait for the import to finish": "请等待导入完成"
 }
diff --git a/crates/ui-egui/locales/zh-hant.json b/crates/ui-egui/locales/zh-hant.json
index 3d179de54..f7e7c35e7 100644
--- a/crates/ui-egui/locales/zh-hant.json
+++ b/crates/ui-egui/locales/zh-hant.json
@@ -1759,5 +1759,7 @@
   "Failed": "失敗",
   "Stopped": "已停止",
   "Stopped by an error (see the log)": "因錯誤而停止（請見記錄）",
-  "Not saved": "未儲存"
+  "Not saved": "未儲存",
+  "A folder is being synchronized": "正在同步資料夾",
+  "Wait for the import to finish": "請等待匯入完成"
 }
diff --git a/crates/ui-egui/src/import.rs b/crates/ui-egui/src/import.rs
index 49e3a2664..4b1e40770 100644
--- a/crates/ui-egui/src/import.rs
+++ b/crates/ui-egui/src/import.rs
@@ -393,6 +393,12 @@ pub struct ScanTask {
     sources: Vec<String>,
 }
 
+/// An import and a Synchronize Folder never run at once: each readies its files against the
+/// library as it was when it started, so both could add the same file.
+pub(crate) fn busy_synchronizing(app: &LightcraftApp) -> Result<(), String> {
+    if app.sync_run.is_some() { Err(crate::i18n::tr("A folder is being synchronized").to_string()) } else { Ok(()) }
+}
+
 /// Scan `paths` in the background, then open the review dialog (see [`poll_scan`]).
 pub fn open(app: &mut LightcraftApp, paths: Vec<String>) -> Result<Value, String> {
     if app.scan.is_some() {
@@ -565,6 +571,7 @@ impl ScanTask {
 
 /// Start importing the dialog's checked files (the dialog's OK / `ui.dialog.confirm`).
 pub fn start(app: &mut LightcraftApp, d: &ImportDialog) -> Result<Value, String> {
+    busy_synchronizing(app)?;
     let queue = d.selected_paths();
     if queue.is_empty() {
         return Err("no photos selected".into());
@@ -616,6 +623,7 @@ pub fn start(app: &mut LightcraftApp, d: &ImportDialog) -> Result<Value, String>
 
 /// Start importing `paths` (files or folders) in the background, e.g. dropped on the window.
 pub fn start_paths(app: &mut LightcraftApp, paths: Vec<String>) -> Result<Value, String> {
+    busy_synchronizing(app)?;
     if app.import.is_some() || app.scan.as_ref().is_some_and(|t| !t.browse) {
         return Err("an import is running".into());
     }
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 503c50b28..05591366a 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -772,7 +772,12 @@ impl LightcraftApp {
         if let Some(folder) = self.session.import_defaults.auto_folder.clone() {
             const LABEL: &str = "Auto Import";
             let now = ctx.input(|i| i.time);
-            if now - self.ui.auto_import_at >= 3.0 && self.import.is_none() && self.lightroom.is_none() && !self.tasks.is_running(LABEL) {
+            if now - self.ui.auto_import_at >= 3.0
+                && self.import.is_none()
+                && self.sync_run.is_none()
+                && self.lightroom.is_none()
+                && !self.tasks.is_running(LABEL)
+            {
                 self.ui.auto_import_at = now;
                 let work = move || lightcraft_engine::cmd::library::list_auto_import_folder(&folder);
                 let done = |app: &mut LightcraftApp, _ctx: &egui::Context, listing: Result<Vec<(String, u64)>, String>| {
diff --git a/crates/ui-egui/src/sync.rs b/crates/ui-egui/src/sync.rs
index b60243ac1..244f4308c 100644
--- a/crates/ui-egui/src/sync.rs
+++ b/crates/ui-egui/src/sync.rs
@@ -172,6 +172,10 @@ pub fn confirm(app: &mut LightcraftApp, dlg: &Dialog) -> Result<Value, String> {
     if app.sync_run.is_some() {
         return Err(crate::i18n::tr("A folder is already being synchronized").to_string());
     }
+    // (an import readies its files against the library as it was: both could add the same file)
+    if app.import.is_some() {
+        return Err(crate::i18n::tr("Wait for the import to finish").to_string());
+    }
     let choice = SyncChoice { import_new: *import_new, relink_moved: *relink_moved, remove_missing: *remove_missing, read_metadata: *read_metadata };
     app.sync_owns_changes = false;
     let started = match app.session.take_folder_changes(path) {
diff --git a/crates/ui-egui/src/tests_sync.rs b/crates/ui-egui/src/tests_sync.rs
index dff960bc9..59fd041f5 100644
--- a/crates/ui-egui/src/tests_sync.rs
+++ b/crates/ui-egui/src/tests_sync.rs
@@ -217,3 +217,32 @@ fn a_stopped_synchronize_says_it_was_stopped_and_stays_at_most_one_step() {
     assert!(toast.contains("Stopped"), "never 'up to date': {toast:?}");
     assert!(h.app.session.undo.len() <= steps + 1, "what was done by then is at most one step");
 }
+
+#[test]
+fn a_synchronize_and_an_import_never_run_at_once() {
+    let dir = Scratch::new("exclusive");
+    let mut h = changed_folder(&dir);
+    open_and_scan(&mut h, &dir.path("trip"));
+    let dlg = h.app.ui.dialog.take().expect("the dialog is open");
+    assert!(crate::panels::dialogs::confirm_dialog(&mut h.app, &dlg).is_ok());
+    // dropping the same folder on the window while the run works would add its files twice
+    let r = crate::import::start_paths(&mut h.app, vec![dir.path("trip")]);
+    assert!(r.is_err(), "no import while synchronizing: {r:?}");
+    assert!(h.step_until(T, |h| h.app.sync_run.is_none()));
+    // and the other way round: an import is running (no frame has run since it started)
+    let r = crate::import::start_paths(&mut h.app, vec![dir.path("trip/a.png")]);
+    assert!(r.is_ok(), "{r:?}");
+    assert!(h.app.import.is_some());
+    let d = Dialog::SynchronizeFolder {
+        path: dir.path("trip"),
+        name: "trip".into(),
+        disk: false,
+        counts: Some(Default::default()),
+        import_new: true,
+        relink_moved: true,
+        remove_missing: false,
+        read_metadata: false,
+    };
+    let r = crate::panels::dialogs::confirm_dialog(&mut h.app, &d);
+    assert!(r.is_err() && h.app.sync_run.is_none(), "no synchronize while importing: {r:?}");
+}