Use unique id for Undo (#8434)

packages/backend/src/remote/activitypub/renderer/undo.ts

@@ -3,9 +3,11 @@ import { ILocalUser, User } from '@/models/entities/user.js';
 
 export default (object: any, user: { id: User['id'] }) => {
 	if (object == null) return null;
+	const id = typeof object.id === 'string' && object.id.startsWith(config.url) ? `${object.id}/undo` : undefined;
 
 	return {
 		type: 'Undo',
+		...(id ? { id } : {}),
 		actor: `${config.url}/users/${user.id}`,
 		object,
 		published: new Date().toISOString(),