What do I do when somebody claims my son two years in a row that we didn’t have permission to do so is not the father is not a family member

[Opus 17]

Prepare a correct tax return that claims the dependents if you are allowed to.  Sign and mail it.  The IRS will investigate any duplicate claim.  You may be asked to provide proof that you are the person who is qualified to claim them.  You can also file an amended return for last year to claim them if you were prevented last year.

You can also request an IP PIN (identify protection PIN) from the IRS for a minor child.  This is a 6 digit PIN that the IRS resets every year, and you can't claim the dependent without the current IP PIN (it will be mailed to the person or it can be retrieved on line). https://www.irs.gov/identity-theft-fraud-scams/get-an-identity-protection-pin

Lastly, if your child is the victim of identity theft, there are other steps you can take. 

https://www.identitytheft.gov/

[Hal_Al]

If someone else claimed your child inappropriately, and if they file first, your return will be rejected if e-filed. You would then need to file a return on paper, claiming the child as  appropriate. The IRS will process your return and send you your refund, in the normal time. Shortly (up to a year) thereafter, you'll receive a letter from the IRS, stating that your child was claimed on another return. It will tell you that if you made a mistake to file an amended return and if you didn't make a mistake to do nothing. The other party will get the same letter you did. If one of you doesn't file an amended return, unclaiming the child, the next letter, from the IRS, will require you to provide proof. Be sure to reply in a timely manner.

Winner gets the tax benefits; loser gets to pay the IRS back with penalties and interest.  The custodial parent almost always wins.