players having trouble logging in from Lobby

GM: Macbook Pro, OS X 10.13.6, D20pro 3.7.3 (stable release)
Players: windows machines various OS mostly the latest window and one windows vista (I think)
The players involved (there are 3) each receive a message there is someone already logged on with their username.
The work around to this point is to log off and then back on with a slightly different name ex. character A becomes character A2. This has worked but even so it may take A3 or even A4 before the player logs in. Now, often but not always, I will see each of the iterations of the character name show up in the game log. They usually stay there the entire session, taking up a seat, unless I as the GM shut down and restart.

Is there something I am not doing correctly to cause these problems for my gamers? Any help is appreciated.
Mickey

 

I don't have that on the GM machine. I'll check with the players and report back. Thanks.