diff --git a/README.md b/README.md
index 1a6ad2e..bbdd6be 100644
--- a/README.md
+++ b/README.md
@@ -78,6 +78,14 @@ bash scripts/e2e.sh
 # or: make e2e
 ```
 
+## Known limitations
+
+Deduplication key is `UNIQUE(account_id, message_key)` **without folder**. If
+the same message appears in multiple source folders (e.g. Gmail `INBOX` +
+`[Gmail]/All Mail` + labels-as-folders), it is copied only into whichever
+destination folder is processed first; folder placement for such duplicated
+messages is not guaranteed. This is intentional per the design spec.
+
 ## Architecture
 
 - `cmd/server` — entrypoint: runs DB migrations, then serves HTTP.
diff --git a/internal/httpapi/run.go b/internal/httpapi/run.go
index bbedaab..b9b689a 100644
--- a/internal/httpapi/run.go
+++ b/internal/httpapi/run.go
@@ -74,6 +74,10 @@ func (s *Server) handleRun(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "accounts must pass connection tests first", http.StatusConflict)
 		return
 	}
+	if errors.Is(err, orchestrator.ErrAlreadyRunning) {
+		http.Error(w, "task is already running", http.StatusConflict)
+		return
+	}
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
diff --git a/internal/orchestrator/orchestrator.go b/internal/orchestrator/orchestrator.go
index efbebdd..b693a5a 100644
--- a/internal/orchestrator/orchestrator.go
+++ b/internal/orchestrator/orchestrator.go
@@ -13,6 +13,7 @@ import (
 )
 
 var ErrNotTested = errors.New("accounts not fully tested")
+var ErrAlreadyRunning = errors.New("task already running")
 
 type Orchestrator struct {
 	store       *store.Store
@@ -99,15 +100,23 @@ func (o *Orchestrator) Run(ctx context.Context, taskID int64) (int64, error) {
 	if !gateOK(accs) {
 		return 0, ErrNotTested
 	}
+	acquired, err := o.store.TryMarkTaskRunning(ctx, taskID)
+	if err != nil {
+		return 0, err
+	}
+	if !acquired {
+		return 0, ErrAlreadyRunning
+	}
 	srcEP, dstEP, err := o.endpoints(ctx, task)
 	if err != nil {
+		_ = o.store.SetTaskStatus(ctx, taskID, "error")
 		return 0, err
 	}
 	runID, err := o.store.CreateRun(ctx, taskID)
 	if err != nil {
+		_ = o.store.SetTaskStatus(ctx, taskID, "error")
 		return 0, err
 	}
-	_ = o.store.SetTaskStatus(ctx, taskID, "running")
 	o.hub.Publish(wshub.Event{Type: "run_started", TaskID: taskID, Data: map[string]any{"run_id": runID}})
 
 	go o.runAll(context.WithoutCancel(ctx), task, runID, accs, srcEP, dstEP)
@@ -138,8 +147,12 @@ func (o *Orchestrator) runAll(ctx context.Context, task store.Task, runID int64,
 	}
 	wg.Wait()
 
-	_ = o.store.FinishRun(ctx, runID, "done", totCopied, totSkipped, totErr)
-	_ = o.store.SetTaskStatus(ctx, task.ID, "done")
+	status := "done"
+	if totErr > 0 {
+		status = "done_with_errors"
+	}
+	_ = o.store.FinishRun(ctx, runID, status, totCopied, totSkipped, totErr)
+	_ = o.store.SetTaskStatus(ctx, task.ID, status)
 	o.hub.Publish(wshub.Event{Type: "run_done", TaskID: task.ID,
 		Data: map[string]any{"run_id": runID, "copied": totCopied, "skipped": totSkipped, "errors": totErr}})
 }
diff --git a/internal/store/tasks.go b/internal/store/tasks.go
index c452c13..f4b13d3 100644
--- a/internal/store/tasks.go
+++ b/internal/store/tasks.go
@@ -55,3 +55,14 @@ func (s *Store) SetTaskStatus(ctx context.Context, id int64, status string) erro
 	_, err := s.Pool.Exec(ctx, `UPDATE tasks SET status=$2 WHERE id=$1`, id, status)
 	return err
 }
+
+// TryMarkTaskRunning atomically sets status='running' only if the task is not already running.
+// Returns true if this call acquired the run (status was not 'running' before), false otherwise.
+func (s *Store) TryMarkTaskRunning(ctx context.Context, id int64) (bool, error) {
+	ct, err := s.Pool.Exec(ctx,
+		`UPDATE tasks SET status='running' WHERE id=$1 AND status<>'running'`, id)
+	if err != nil {
+		return false, err
+	}
+	return ct.RowsAffected() == 1, nil
+}
diff --git a/internal/store/tasks_test.go b/internal/store/tasks_test.go
new file mode 100644
index 0000000..78346b0
--- /dev/null
+++ b/internal/store/tasks_test.go
@@ -0,0 +1,32 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+func TestTryMarkTaskRunningIsExclusive(t *testing.T) {
+	s := testStore(t)
+	ctx := context.Background()
+	epSrc, _ := s.CreateEndpoint(ctx, Endpoint{RoleLabel: "src", Host: "a", Port: 993, TLSMode: "ssl"})
+	epDst, _ := s.CreateEndpoint(ctx, Endpoint{RoleLabel: "dst", Host: "b", Port: 993, TLSMode: "ssl"})
+	taskID, _ := s.CreateTask(ctx, Task{Name: "t", SrcEndpointID: epSrc, DstEndpointID: epDst})
+
+	first, err := s.TryMarkTaskRunning(ctx, taskID)
+	if err != nil || !first {
+		t.Fatalf("first acquire must succeed: ok=%v err=%v", first, err)
+	}
+	second, err := s.TryMarkTaskRunning(ctx, taskID)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if second {
+		t.Fatal("second acquire must fail while running")
+	}
+	// after completion, a re-run may acquire again
+	_ = s.SetTaskStatus(ctx, taskID, "done")
+	third, _ := s.TryMarkTaskRunning(ctx, taskID)
+	if !third {
+		t.Fatal("acquire after completion must succeed")
+	}
+}
diff --git a/web/src/pages/TaskDetail.tsx b/web/src/pages/TaskDetail.tsx
index 788de4f..074fd41 100644
--- a/web/src/pages/TaskDetail.tsx
+++ b/web/src/pages/TaskDetail.tsx
@@ -152,7 +152,7 @@ export function TaskDetail({ id }: { id: number }) {
           <button className="btn" onClick={onTest} disabled={busy !== null || accounts.length === 0}>
             {busy === 'test' ? 'Testing…' : 'Test connections'}
           </button>
-          <button className="btn btn-primary" onClick={onRun} disabled={busy !== null || !allTested}>
+          <button className="btn btn-primary" onClick={onRun} disabled={busy !== null || !allTested || task.status === 'running'}>
             {busy === 'run' ? 'Starting…' : 'Run migration'}
           </button>
           {!allTested && accounts.length > 0 && <span className="hint">run unlocks once every account tests OK on both sides</span>}