Messages being sent by friends are doubling, and the chat window is not scrolling down automatically. This only happens in pop-out mode, I switched to test embedded and the issue was not occurring there.
Issue was still present in pop-out after switching to beta, then fully uninstalling and reinstalling the regular client. Also restarted computer and modem.
When the window is closed, and then re-opened, the doubled “ghost” messages disappear, but any new messages sent afterwards continues doubling again.
It happens whether friend is in-game or not. I asked about 6 people, only one is having the same issue (different location, different ISP). Customer service team on twitter could not replicate the problem.