Implemented behavior where logged in users are tracked, and if they log
in a second time, an exception is thrown. This is because a user logging
into our system twice is bad. It's also probably not a good idea in the
protocol, that seems problematic when playing blackjack.
Checks if a user is already logged in, and doesn't allow them to log in
twice. This required logging out users when their associated thread ends
for whatever reason.