Two returns cannot claim the same person. The IRS only cares about physical custody. Whoever has custody more than 50.01 % of the time is entitled to claim the deduction. One of you will need to amend the return.
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Typically, the first return to claim the dependent is assumed to be correct, and anyone else is not allowed to claim the dependent, regardless of the situation. This happens a lot with parents who share custody. Of course, you should only claim a dependent that you have the right to claim for that year.
FIrst to file generally gets the exemption, although the rightful claimant will eventually prevail. I have seen the IRS disallow both and force the parents to prove entitlement too.
If two people claim the same dependent, sometimes legally, the IRS will use "tiebreaker rules" to determine who gets to claim the dependent. These rules only apply when more than one person claims a dependent. For example, if two parents have a child more than 50.1 percent of the year (let's say they separate in August), and they both claim the child, then the parent with the highest AGI will be able to claim the child under the tiebreaker rule.
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