Open Radar

 
Area:
Messages

Summary:
Using Jabber with Messages results in repeated messages showing up.

Here's a rough ASCII drawing of the issue, since I don't have a screenshot (my messages on the right):
--------------------------------
|                 | Message 1 || <--------- Chat window
|| Message 2 |                 |
|| Message 3 |                 |
|                 | Message 4 ||
|                 | Message 5 ||
|| Message 6 |                 |
|                 | Message 1 ||
|                 | Message 4 ||
|                 | Message 5 ||
|                              |
|------------------------------|
|Jabber (from Slack) 😃        | <------------ Place to type messages
|------------------------------|

Notice that Messages 1, 4, 5 are in the correct place in the conversation, but are repeated again at the bottom. The duplicates go away if the chat is closed and reopened.

Steps to Reproduce:
1. Connect to a Jabber chat room (for example, Slack)
2. Send a couple messages
3. See duplicates

Expected Results:
No duplicates at the bottom

Actual Results:
Duplicates of my messages show up at the bottom

Version:
macOS Sierra 10.12.4 Beta (16E175b)

Notes:
Someone on the support forums has the same issue, with screenshots and more information:
https://discussions.apple.com/thread/6638767?start=0&tstart=0

Configuration:
Early 2015 MacBook Pro with Retina Display running macOS Sierra 10.12.4 Beta (16E175b)