This happened a few years back, but we filed first (I do our taxes on line with TurboTax by the first week of Feb.), like usual. For reasons we couldn't fathom,
BM was adamant that
DH wasn't current with CS at the end of the year, thus he could not take the deduction. It took us until almost May to finally convince her DH WAS current (had to get a notarized letter from CSRU for her to believe it).
By that time, we knew she had filed and claimed
SS, too. But we didn't get notification of it until 2 days before Christmas. Got the same letter, too...........SS's SSN had been used on two different returns. But the letter also said that if we had filed correctly, we didn't have to respond. So we didn't. Then I called a local IRS office, actually talked to a 'human being' and asked what we should do in the coming year. Quote her: 'He who files first, gets.' Apparently the IRS will iron it all out later if there's any discrepancies. She could not tell me what they would or could do to BM, but I am assuming either she had to pay it back or they deducted it from a future return.
I doubt there were any penalties, because I know we would have heard about it, either directly or indirectly from BM. Funny thing is, she tried to claim DH was late, when CS was being garnished! Apparently, CSRU calculated receipt when they received it from DH's employer and BM was trying to calculate it when it hit her bank account, LOL! ..........And we've never had a problem with it since!
I doubt that you will be able to find out what any repercussions will be, unless the BM tells you herself. But depending on what those repercussions are, you have more assurance that there's less likelihood she'll do it again. I do strongly recommend that from now on, you file as soon as possible, though. That way you won't have to worry about what she files and you won't have to wait for the IRS to iron things out if she tries it again.