I am UIT and I joined the Serco military. When I docked at an Itani station, in the mission list was a mission for joining Itani military. I suppose it is a bug, coz i was told that I cannot join the opposite nation when I have already joined the Serco military.

Yeah, it works in reverse too. I still see offers to join the Serco military even though I'm in the Itani one.