I have the same concern. According to my attorney, the USCIS will issue a RFE and request you to withdraw one I485 before the adjudication.
But according to Xiao Bitu, the USCIS will approve one and withdraw another one for you.
I guess we need Xia Bitu's advice definitely.
Please Xiao Baitu, can you give us a clear answer?
thanks
-P