diff --git a/lib/services/microsoft-calendar-sync.ts b/lib/services/microsoft-calendar-sync.ts
index d8ca774..7f4c832 100644
--- a/lib/services/microsoft-calendar-sync.ts
+++ b/lib/services/microsoft-calendar-sync.ts
@@ -507,7 +507,8 @@ export async function syncMicrosoftCalendar(
 
       // Priority 3: Fallback to title + date matching for events without externalEventId
       // IMPORTANT: Only match if the event doesn't have an externalEventId (to avoid false matches)
-      if (!existingEvent) {
+      // This helps migrate old events that were created before externalEventId was added
+      if (!existingEvent && microsoftId) {
         existingEvent = existingEvents.find(
           (e) => {
             // Access externalEventId safely (may not be in Prisma type if client not regenerated)
@@ -521,10 +522,11 @@ export async function syncMicrosoftCalendar(
             if (e.title === caldavEvent.summary) {
               const timeDiff = Math.abs(new Date(e.start).getTime() - caldavEvent.start.getTime());
               if (timeDiff < 60000) { // Within 1 minute
-                logger.debug('Matched event by title + date (no externalEventId)', {
+                logger.debug('Matched event by title + date (no externalEventId) - will update with externalEventId', {
                   eventId: e.id,
                   title: caldavEvent.summary,
                   timeDiff,
+                  microsoftId,
                 });
                 return true;
               }
diff --git a/scripts/cleanup-duplicate-events.ts b/scripts/cleanup-duplicate-events.ts
new file mode 100644
index 0000000..4474760
--- /dev/null
+++ b/scripts/cleanup-duplicate-events.ts
@@ -0,0 +1,145 @@
+#!/usr/bin/env ts-node
+
+/**
+ * Script to clean up duplicate events in the database
+ * Keeps events with externalEventId, removes duplicates without externalEventId
+ */
+
+import { PrismaClient } from '@prisma/client';
+
+const prisma = new PrismaClient();
+
+interface DuplicateGroup {
+  title: string;
+  start: Date;
+  end: Date;
+  calendarId: string;
+  events: Array<{
+    id: string;
+    externalEventId: string | null;
+  }>;
+}
+
+async function cleanupDuplicateEvents() {
+  try {
+    console.log('🔍 Searching for duplicate events...\n');
+
+    // Find all events grouped by title, start, end, and calendarId
+    const allEvents = await prisma.event.findMany({
+      select: {
+        id: true,
+        title: true,
+        start: true,
+        end: true,
+        calendarId: true,
+        externalEventId: true,
+      },
+      orderBy: [
+        { calendarId: 'asc' },
+        { title: 'asc' },
+        { start: 'asc' },
+      ],
+    });
+
+    // Group events by title + start + end + calendarId
+    const eventGroups = new Map<string, DuplicateGroup>();
+
+    for (const event of allEvents) {
+      const key = `${event.calendarId}|${event.title}|${event.start.toISOString()}|${event.end.toISOString()}`;
+      
+      if (!eventGroups.has(key)) {
+        eventGroups.set(key, {
+          title: event.title,
+          start: event.start,
+          end: event.end,
+          calendarId: event.calendarId,
+          events: [],
+        });
+      }
+      
+      eventGroups.get(key)!.events.push({
+        id: event.id,
+        externalEventId: event.externalEventId,
+      });
+    }
+
+    // Find duplicates (groups with more than 1 event)
+    const duplicates: DuplicateGroup[] = [];
+    for (const group of eventGroups.values()) {
+      if (group.events.length > 1) {
+        duplicates.push(group);
+      }
+    }
+
+    console.log(`Found ${duplicates.length} groups of duplicate events\n`);
+
+    if (duplicates.length === 0) {
+      console.log('✅ No duplicates found!');
+      return;
+    }
+
+    let totalDeleted = 0;
+    let totalKept = 0;
+
+    for (const group of duplicates) {
+      console.log(`\n📋 Processing: "${group.title}" (${group.events.length} duplicates)`);
+      console.log(`   Calendar: ${group.calendarId}`);
+      console.log(`   Date: ${group.start.toISOString()}`);
+
+      // Separate events with and without externalEventId
+      const withExternalId = group.events.filter(e => e.externalEventId);
+      const withoutExternalId = group.events.filter(e => !e.externalEventId);
+
+      if (withExternalId.length > 1) {
+        // Multiple events with externalEventId - keep the first one, delete others
+        console.log(`   ⚠️  Warning: Multiple events with externalEventId found!`);
+        const toKeep = withExternalId[0];
+        const toDelete = [...withExternalId.slice(1), ...withoutExternalId];
+        
+        console.log(`   ✅ Keeping: ${toKeep.id} (has externalEventId)`);
+        for (const event of toDelete) {
+          console.log(`   🗑️  Deleting: ${event.id}`);
+          await prisma.event.delete({ where: { id: event.id } });
+          totalDeleted++;
+        }
+        totalKept++;
+      } else if (withExternalId.length === 1) {
+        // One event with externalEventId - keep it, delete others
+        const toKeep = withExternalId[0];
+        const toDelete = withoutExternalId;
+        
+        console.log(`   ✅ Keeping: ${toKeep.id} (has externalEventId)`);
+        for (const event of toDelete) {
+          console.log(`   🗑️  Deleting: ${event.id}`);
+          await prisma.event.delete({ where: { id: event.id } });
+          totalDeleted++;
+        }
+        totalKept++;
+      } else {
+        // No events with externalEventId - keep the first one, delete others
+        const toKeep = group.events[0];
+        const toDelete = group.events.slice(1);
+        
+        console.log(`   ✅ Keeping: ${toKeep.id} (no externalEventId, keeping first)`);
+        for (const event of toDelete) {
+          console.log(`   🗑️  Deleting: ${event.id}`);
+          await prisma.event.delete({ where: { id: event.id } });
+          totalDeleted++;
+        }
+        totalKept++;
+      }
+    }
+
+    console.log(`\n✅ Cleanup completed!`);
+    console.log(`   Kept: ${totalKept} events`);
+    console.log(`   Deleted: ${totalDeleted} duplicate events`);
+
+  } catch (error) {
+    console.error('❌ Error cleaning up duplicates:', error);
+    throw error;
+  } finally {
+    await prisma.$disconnect();
+  }
+}
+
+cleanupDuplicateEvents();