"Bad callsign - already taken" when joining Multiplayer

[SantasGolfClub]

I am trying to learn with an online mate, and I started a server on my local machine. I opened the port (Not sure if I have to, but I  did), and sent him the IP. Because it was public, he actually saw the server on the server browser.

But as soon as he tried to join, he got the message: Bad callsign - already taken

Someone else (A random) did manage to join. So I believe it's the other chap trying to connect to me that has the issue. Has anyone seen this and know what the fix is?