If a dependent that you are entitled to claim has already been claimed by another taxpayer, your e-filed return will reject since the child's SSN has already been used (either intentionally or in error such as a mistyped SSN).
Your only recourse is to file a correct tax return, claiming what you are entitled to claim, then print and mail the return.
The IRS will process both returns and pay any refunds. Shortly (within a year) the IRS will mail letters to both taxpayers asking if their tax return was filed in error and suggesting that they amend if they improperly claimed the child.
If neither taxpayer amends, the IRS will send a second letter asking for each taxpayers proof that they are entitled to claim the dependent, such as proof that the child physically lived with them more than half the year. School records, child care records, household receipts, medical bills, etc., that show that the child lives with you should be retained.
The IRS will evaluate each taxpayers claim and award the dependent to one taxpayer, the other will have to payback any refund received plus interest and possible penalties. The parent that had physical custody usually always wins.
Do not ignore the letters or you will loose.
You could file an identity theft report with the IRS if you know for a fact that a tax return was fraudulently filed with their SS number on the return as a dependent. Use IRS Form 14039, Identity Theft Affidavit - https://www.irs.gov/pub/irs-pdf/f14039.pdf
IRS website for identity theft information - https://www.irs.gov/newsroom/taxpayer-guide-to-identity-theft
If your e-filed tax return was rejected by the IRS because of another tax return being filed with the SS number and name of your dependent on that tax return, then you will have to print and mail the tax return and include the dependent's information on your tax return. If you are owed a tax refund the IRS will pay the refund.
See this TurboTax support FAQ for the procedure to print a tax return for mailing when using the online editions - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...
Within a year the IRS will contact both parties to investigate who was eligible to claim the dependent on their tax return. The loser of the investigation will have to pay back any tax refund plus any penalties and interest owed for improperly claiming the dependent.
When you are contacted by the IRS by mail do not ignore the request made by the IRS for information.