to clarify if you file first your return will be accepted. the other taxpayer's return claiming the same dependent(s) will be rejected and they will have to mail it in. you will not be notified of this. at some point both of you will get an IRS letter requesting each to amend to take off the dependents. this is when you'll first know someone else claimed one or more of your dependents. if neither amends there is a second letter asking each to provide proof that they are the one entitled to claim the dependents. the one that provides the stronger proof gets to keep them. the other gets a bill.
