2 files changed, 44 insertions, 2 deletions
diff --git a/internal/store/native_tasks.go b/internal/store/native_tasks.go
index 3bd87cb..90cf275 100644
--- a/internal/store/native_tasks.go
+++ b/internal/store/native_tasks.go
@@ -228,6 +228,10 @@ func (s *Store) UncompleteNativeTask(id string) error {
 // same recurrence_series_id, completed=false, next_occurrence_override=""),
 // with due_date set to old.NextOccurrenceOverride if present, else
 // ComputeNextOccurrence(old.DueDate, ...). old itself is left untouched.
+// The INSERT is self-guarding: it atomically no-ops if a newer row already
+// exists in the series, so two concurrent triggers (the completion path and
+// the periodic due-date check) can never both create a successor for the
+// same predecessor.
 func (s *Store) CreateNextIteration(old models.Task) error {
 	var nextDue *time.Time
 	switch {
@@ -244,9 +248,16 @@ func (s *Store) CreateNextIteration(old models.Task) error {
 			id, content, description, project_name, due_date, priority, labels,
 			recurrence_freq, recurrence_interval, recurrence_weekdays, recurrence_series_id, next_occurrence_override,
 			created_at, updated_at
-		) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, '', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
+		)
+		SELECT ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, '', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP
+		WHERE NOT EXISTS (
+			SELECT 1 FROM native_tasks
+			WHERE recurrence_series_id = ?
+			  AND (due_date > ? OR (due_date = ? AND created_at > ?))
+		)
 	`, newTaskID(), old.Content, old.Description, old.ProjectName, nextDue, old.Priority, string(labelsJSON),
-		old.RecurrenceFreq, old.RecurrenceInterval, formatWeekdays(old.RecurrenceWeekdays), old.RecurrenceSeriesID)
+		old.RecurrenceFreq, old.RecurrenceInterval, formatWeekdays(old.RecurrenceWeekdays), old.RecurrenceSeriesID,
+		old.RecurrenceSeriesID, old.DueDate, old.DueDate, old.CreatedAt)
 	return err
 }
 
diff --git a/internal/store/native_tasks_test.go b/internal/store/native_tasks_test.go
index a299ac6..c362eb7 100644
--- a/internal/store/native_tasks_test.go
+++ b/internal/store/native_tasks_test.go
@@ -302,6 +302,37 @@ func TestCompleteNativeTask_AlreadySuperseded_DoesNotDoubleCreate(t *testing.T)
 	}
 }
 
+func TestCreateNextIteration_ConcurrentGuard_SecondCallIsNoOp(t *testing.T) {
+	s := newNativeTasksTestStore(t)
+	if _, err := s.db.Exec(`
+		INSERT INTO native_tasks (id, content, due_date, recurrence_freq, recurrence_interval, recurrence_series_id)
+		VALUES ('rec-1', 'Water plants', '2026-07-13', 'weekly', 1, 'series-1')
+	`); err != nil {
+		t.Fatal(err)
+	}
+	old, err := s.GetNativeTaskByID("rec-1")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Simulate two concurrent triggers both computing from the same
+	// snapshot of rec-1 and both attempting to create its successor.
+	if err := s.CreateNextIteration(*old); err != nil {
+		t.Fatalf("first CreateNextIteration: %v", err)
+	}
+	if err := s.CreateNextIteration(*old); err != nil {
+		t.Fatalf("second CreateNextIteration (should be a safe no-op, not an error): %v", err)
+	}
+
+	var count int
+	if err := s.db.QueryRow(`SELECT COUNT(*) FROM native_tasks WHERE recurrence_series_id = 'series-1' AND id != 'rec-1'`).Scan(&count); err != nil {
+		t.Fatal(err)
+	}
+	if count != 1 {
+		t.Errorf("expected exactly 1 successor row after two CreateNextIteration calls from the same snapshot, got %d", count)
+	}
+}
+
 func TestSetTaskRecurrence_FirstTimeGeneratesSeriesID(t *testing.T) {
 	s := newNativeTasksTestStore(t)