Can you activate a system that you already did a mission in? i havent found anything clarifying this. i know it says that you cant activate a system that has a person in it but it also goes back and says that you cant move from a planet that has been activated not just with a person in it. thanks!

I think so, i know it says you cannot move units OUT of a system that contains a leader.

You can activate a system with a leader in it. The only thing leaders limit in regards to activating a system is moving units OUT of a system with a friendly leader in it.