I believe this is something we never thought could happen because at the start DMP synced so fast. However, on servers with a long load time (due to the millions of ships people insist on spawning, and the millions of kerbals that spawn along with them), if two players try to connect at the same time it causes the handshake to fail, with no explanation.

It should give you a warning that someone else is connecting, and that you're x in line to sync.

Similar to Battlefield's queue system, you join, and get queued. If you're first in the queue, you sync, and join. If you're 7th, you'll have to wait.

Can we confirm that this exists? - Basically, when a client connects, it gets setup and added into the ClientObject list (clients). There shouldn't be any reason why multiple players can't connect at once :-/

EDIT: Rocky, if you're interested in the trace:
setup server
accept client
actual setup and enqueue
where client enters the actual client list and becomes part of the message relaying

There's nothing really around that would prevent 2 clients from syncing :-/

Shouldn't be a problem now, especially with the immutable client list change.