Shouldn't matter if a user is messaging from a separate tab or device...

under the same user name, chat history should always broadcast.
This needs to be tested -- my guess is it will create duplicates of the message for the user who posts it (because we have the message show up in the chat window before it is "confirmed" at the server level, at which point the message would then post again.

This line could probably just be simplified to if !activityExists