Prevent duplication of mention on post update

and delete mentions and notifications of user no longer mentioned

src/routes/posts.rs

@@ -210,8 +210,15 @@ fn update(blog: String, slug: String, user: User, conn: DbConn, data: LenientFor
             let post = post.update_ap_url(&*conn);
 
             if post.published {
-                for m in mentions.into_iter() {
-                    Mention::from_activity(&*conn, Mention::build_activity(&*conn, m), post.id, true, true);
+                let old_mentions = Mention::list_for_post(&conn, post.id);
+                let old_user_mentioned = old_mentions.iter()
+                    .filter_map(|m| User::get(&conn, m.mentioned_id).map(|u| u.get_fqn(&conn)))
+                    .collect::<HashSet<_>>();
+                for m in mentions.difference(&old_user_mentioned).into_iter() {
+                    Mention::from_activity(&*conn, Mention::build_activity(&*conn, m.clone()), post.id, true, true);
+                }
+                for m in old_mentions.iter().filter(|m| !User::get(&conn, m.mentioned_id).map(|u| mentions.contains(&u.get_fqn(&conn))).unwrap_or(false)) {
+                    m.delete(&conn);
                 }
             }