diff --git a/backend/src/activitypub/activities/handle.ts b/backend/src/activitypub/activities/handle.ts
index 8462492..5c2c0f9 100644
--- a/backend/src/activitypub/activities/handle.ts
+++ b/backend/src/activitypub/activities/handle.ts
@@ -25,6 +25,7 @@ import { createReblog } from 'wildebeest/backend/src/mastodon/reblog'
 import { insertReply } from 'wildebeest/backend/src/mastodon/reply'
 import type { Activity } from 'wildebeest/backend/src/activitypub/activities'
 import { originalActorIdSymbol } from 'wildebeest/backend/src/activitypub/objects'
+import { hasReblog } from 'wildebeest/backend/src/mastodon/reblog'
 
 function extractID(domain: string, s: string | URL): string {
 	return s.toString().replace(`https://${domain}/ap/users/`, '')
@@ -289,6 +290,12 @@ export async function handle(
 
 			const fromActor = await actors.getAndCache(actorId, db)
 
+			if (await hasReblog(db, fromActor, obj)) {
+				// A reblog already exists. To avoid dulicated reblog we ignore.
+				console.warn('probably duplicated Announce message')
+				break
+			}
+
 			// notify the user
 			const targetActor = await actors.getActorById(db, new URL(obj[originalActorIdSymbol]))
 			if (targetActor === null) {
diff --git a/backend/src/mastodon/reblog.ts b/backend/src/mastodon/reblog.ts
index 5427c7a..2170ad5 100644
--- a/backend/src/mastodon/reblog.ts
+++ b/backend/src/mastodon/reblog.ts
@@ -39,3 +39,12 @@ export function getReblogs(db: D1Database, obj: APObject): Promise<Array<string>
 
 	return getResultsField(statement, 'actor_id')
 }
+
+export async function hasReblog(db: D1Database, actor: Actor, obj: APObject): Promise<boolean> {
+	const query = `
+		SELECT count(*) as count FROM actor_reblogs WHERE object_id=?1 AND actor_id=?2
+	`
+
+	const { count } = await db.prepare(query).bind(obj.id.toString(), actor.id.toString()).first<{ count: number }>()
+	return count > 0
+}
diff --git a/backend/test/activitypub/handle.spec.ts b/backend/test/activitypub/handle.spec.ts
index 55288a3..c41c9c4 100644
--- a/backend/test/activitypub/handle.spec.ts
+++ b/backend/test/activitypub/handle.spec.ts
@@ -545,6 +545,53 @@ describe('ActivityPub', () => {
 				assert(outbox_object)
 				assert.equal(outbox_object.actor_id, remoteActorId)
 			})
+
+			test('duplicated announce', async () => {
+				const remoteActorId = 'https://example.com/actor'
+				const objectId = 'https://example.com/some-object'
+				globalThis.fetch = async (input: RequestInfo) => {
+					if (input.toString() === remoteActorId) {
+						return new Response(
+							JSON.stringify({
+								id: remoteActorId,
+								icon: { url: 'img.com' },
+								type: 'Person',
+							})
+						)
+					}
+
+					if (input.toString() === objectId) {
+						return new Response(
+							JSON.stringify({
+								id: objectId,
+								type: 'Note',
+								content: 'foo',
+							})
+						)
+					}
+
+					throw new Error('unexpected request to ' + input)
+				}
+
+				const db = await makeDB()
+				await createPerson(domain, db, userKEK, 'sven@cloudflare.com')
+
+				const activity: any = {
+					type: 'Announce',
+					actor: remoteActorId,
+					to: [],
+					cc: [],
+					object: objectId,
+				}
+				await activityHandler.handle(domain, activity, db, userKEK, adminEmail, vapidKeys)
+
+				// Handle the same Activity
+				await activityHandler.handle(domain, activity, db, userKEK, adminEmail, vapidKeys)
+
+				// Ensure only one reblog is kept
+				const { count } = await db.prepare('SELECT count(*) as count FROM outbox_objects').first<{ count: number }>()
+				assert.equal(count, 1)
+			})
 		})
 	})
 })