feat: if txn id exists in the db, skip the event

src/client_server/to_device.rs

@@ -20,17 +20,14 @@ pub async fn send_event_to_device_route(
     let sender_user = body.sender_user.as_ref().expect("user is authenticated");
     let sender_device = body.sender_device.as_deref();
 
-    // TODO: uncomment when https://github.com/vector-im/element-android/issues/3589 is solved
     // Check if this is a new transaction id
-    /*
     if db
         .transaction_ids
         .existing_txnid(sender_user, sender_device, &body.txn_id)?
         .is_some()
     {
-        return Ok(send_event_to_device::v3::Response.into());
+        return Ok(send_event_to_device::v3::Response {});
     }
-    */
 
     for (target_user_id, map) in &body.messages {
         for (target_device_id_maybe, event) in map {