Double "user joined" message when changing room with isolate rooms mode

[Et0h]

On changing room the "user joined" message is caused to display twice. It only occurs in isolate rooms mode, so the problem is probably with PublicRoomManager. Reported by Corwin.

[19:31:03] <Bob> has joined the room: 'Foo'
[19:31:07] <Bob> has joined the room: 'Bar'
[19:31:08] <Bob> has joined the room: 'Bar'
[19:32:17] <Alice> has joined the room: 'bar'
[19:32:17] <Alice> has joined the room: 'bar'

[Et0h]

To reproduce join a public server with no file open, then join a second room still with no file open.