duplicate messages when sending - local echo not removed

[xinsight]

I'm seeing duplicate messages when sending from MXKRoomViewController. It seems these are "local echo" messages, which are supposed to be removed after the message sent to the server.

In -[MXKRoomDataSource refreshEventListeners:] - the event listener is created. It checks if there are outgoing messages, and then attempts to find the local echo.

MXEvent *localEcho;
if (self.room.outgoingMessages.count && [event.sender isEqualToString:self.mxSession.myUser.userId])
{
    localEcho = [self.room pendingLocalEchoRelatedToEvent:event];

Two problems: room.outgoingMessages.count is zero. But even if that check is removed, pendingLocalEchoRelatedToEvent: returns nil.

The two messages in the room are almost identical - so i'm not sure how the matrix kit or matrix sdk is supposed to reliably detect the local echo. For example:

{
  "origin_server_ts" : 1597651389299,
  "sender" : "@user1:xxx.org",
  "content" : {
    "msgtype" : "m.text",
    "body" : "Echo?"
  },
  "type" : "m.room.message",
  "event_id" : "$pdATtqY6beNczJlxfpAnk1DbW8k8l9ycTHeeNdij-_0",
  "room_id" : "!rytTykSOlVDqLRpOkK:xxx.org"
}

{
  "sender" : "@user1:xxx.org",
  "content" : {
    "msgtype" : "m.text",
    "body" : "Echo?"
  },
  "origin_server_ts" : 1597651385012,
  "room_id" : "!rytTykSOlVDqLRpOkK:xxx.org",
  "event_id" : "$pdATtqY6beNczJlxfpAnk1DbW8k8l9ycTHeeNdij-_0",
  "unsigned" : {
    "age" : 56854,
    "transaction_id" : "kMXEventLocalId_3AEC3166-F36C-495C-A048-A5FB5D436B56-96982-00019D04955033FC"
  },
  "type" : "m.room.message"
}

Anyone have any thoughts on why the local echo detection might be failing? Any workarounds? This issue happens consistently for me (and I'm not doing anything weird).

[xinsight]

Note: Only one message is sent to the server. If I restart the MXKRoomViewController, I only see each outgoing message a single time (as expected).

[xinsight]

Figured it out. Apparently you need to attach a store to your session. You have MXNoStore, MXMemoryStore or MXFileStore. For example:

let session = MXSession(matrixRestClient: authenticatedRestClient)!
let store = MXMemoryStore()

session.setStore(store) { response in

    guard case .success = response else {
        fatalError("unable to set store")
    }

    session.start { response in
        // ...
    }
}

Would be nice for Matrix SDX to warn that a store was missing! Hopefully this issue can help someone else from some head-scratching.