Hi,

If I play Diplomacy II A) "Choose one system containing a planet you control. Each opponent must place a CC from his reinforcements." as each opponent must place and not activate the system I chose, if an opponent already activated that system it will have 2 CC from that opponent in the chosen system, isn't ?

It will prevent him to play Unexpected Action for example...

I don't know if it's covered in the rule book or FAQ, but that makes sense.