diff --git a/package.json b/package.json
index c4e695a..bbbb992 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "my-weekly-todo-list",
-  "version": "1.66.0",
+  "version": "1.66.1",
   "description": "A web-based weekly task management application that organizes to-dos and calendar events in a single, intuitive weekly view",
   "main": "index.js",
   "scripts": {
diff --git a/src/components/WeeklyView.tsx b/src/components/WeeklyView.tsx
index 1f960f1..2695503 100644
--- a/src/components/WeeklyView.tsx
+++ b/src/components/WeeklyView.tsx
@@ -1613,11 +1613,29 @@ export default function WeeklyView() {
     const [weatherData, setWeatherData] = useState<Record<string, WeatherHour>>({});
 
     // Extend events with editable flag from connections, deduplicate by id
+    // Also deduplicate recurring series masters vs expanded instances:
+    // When a recurring event is created, the master is cached. Then the sync
+    // returns expanded instances with different IDs but the same recurringEventId.
+    // We keep instances and discard masters that overlap with them.
     const calendarEvents = useMemo(() => {
         const seen = new Set<string>();
+        const seenSlot = new Set<string>();
+        // Collect recurring event IDs that have expanded instances
+        const seriesWithInstances = new Set<string>();
+        for (const event of rawCalendarEvents) {
+            if (event.recurringEventId && event.id !== event.recurringEventId) {
+                seriesWithInstances.add(event.recurringEventId);
+            }
+        }
         return rawCalendarEvents.filter((event) => {
             if (seen.has(event.id)) return false;
             seen.add(event.id);
+            // Skip series master if expanded instances exist for this series
+            if (seriesWithInstances.has(event.id)) return false;
+            // Deduplicate by title+startTime+calendarId (catches optimistic add + cache read)
+            const slotKey = `${event.title}|${event.startTime}|${event.calendarId}`;
+            if (seenSlot.has(slotKey)) return false;
+            seenSlot.add(slotKey);
             return true;
         }).map((event) => {
             let isEditable = false;