How can i report someone who claimed a dependent when they were not suppose to?

[Carl]

You don't "report" anything per-se. If the ex claimed the dependent and they were not entitled to (and you are entitled to) then you go ahead and claim them on your tax return. However, you will have to print, sign and mail your return to the IRS this year.

When the IRS receives the return and sees the dependent has been claimed on two returns, the IRS will send each of you a letter requesting that you either:

1) Amend the return to remove the dependent, or
2) Provide specific proof  that you are the one entitled to claim the dependent

The letter will have all the details. Oh also, yes this most likely will hold up your refund for weeks, if not months.

[Tonita]

Please follow the steps in the FAQ below regarding someone claiming your dependent:

https://ttlc.intuit.com/replies/3299938