fix(tasks): make CreateNextIteration atomically self-guarding

Folds the "is this still the latest row in its series?" check directly
into the INSERT as a single atomic INSERT...SELECT...WHERE NOT EXISTS
statement, instead of a separate SELECT-then-INSERT sequence. Closes a
narrow race where the completion trigger and the periodic due-date
check could both pass their "still latest" check before either had
inserted, producing two duplicate successor rows for the same series.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VTUSAEKfsPc6WGDq45yPHD

1 files changed, 13 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
 }