Need help with a sent that only allows one per player

[Mecknavorz]

So i am working on a swep that creates an ent that does stuff, but that is kinda unimportant;
what i need help with is that i am trying to get it were so if i fire the swep again it will remove the previous sent so only one of the sent can be out per player at a time, i am not sure on how to do it, so i am asking for help, does anyone have an idea of what to do?