A addon that makes it so people can't play as the same characters

How would I make a addon so people can't play as the same character?

Keep in mind that this will not be possible if your server has more than 6 people where in an otherwise unmodified game you'd have only 6 playable characters. In any situation where there's more players than characters, the surplus players would either have to use the same character as someone else or be forced to spectate.

In whatever way you wish to implement this, you must use a Lua script. It would be a rather simple one even if you're a Lua beginner, though.

Thanks for the reply. What about making it so only admins can use certain characters? is it possible to do this in the base game without addons?