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That is not correct and the information you've provided is very slim. Why can't you play BF3? You can't join a server or is there another issue? Please elaborate.
You are correct, the issue is coming from both of you connecting to the same server, because you have the same IP. In the past, changing the ports on one of the clients fixed this for me. Try running the game with a different port and see if it works.
Seeing how BF4 has this option and some of its advanced options like HUD and Radar scale already being confirmed to be possible in BF3, it's only a matter of time before such an option appears. That is, if someone makes it, of course