You'll need to paper-file your return, as the IRS will continue to reject your attempts to e-file.
Then, if you know who claimed your dependent, ask them to amend their return by removing the dependent. Amendment processing can take weeks or even months, which is why it's best to paper-file your return in the meantime.
If the other taxpayer is uncooperative, the IRS will eventually contact both of you (by letter) 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.
TurboTax FAQ: What if somebody already claimed my dependent?
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