As far as I know, that join team will be called when the player is connected the first time as a spectator before choosing a team. If you want a dirty fix without checking if the team is T or CT, you can create a bool array of player count, make the index false at client connect and check if the index is false before displaying the message. Then you can display the message and set the index to true.
Actually forget my dirty suggestion. This should have all the info you need:
https://forums.alliedmods.net/showpo...25&postcount=3