There is a process IRS has to deal with this situation in place already.
TurboTax provides questions to help you determine if you have the right to claim a child under these circumstances. If you have the legal right to claim the dependent, there is no issue. File your taxes as normal. If things go well, no news is good news.
"IF" someone has already filed a claim with the IRS using the dependent's Social Security Number the IRS WILL reject any further e-filed claim. These returns must be physically mailed to the IRS.
The IRS will eventually contact both of you to figure out who gets to claim the dependent. The losing party will then be liable for any additional taxes, penalties, and/or interest as a result of the false claim.
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