What happens when both parents claim the same child in the same year on separate income tax returns?

this has happened to my husband for 10 years straight (as of this morning when we got our rejection email).

If the second parent to claim the child e-files, they will get an email from the IRS that their return has been rejected. They will then have to file via paper and snail mail, along with documentation that they are the one who gets that exemption.

If the child lives with one parent more than the other, than that is the parent that can claim the child in the absence of any agreement otherwise. In my husband's divorce agreement, he is entitled to take the exemption EVERY year with no restrictions. But, every year his ex-wife files seemingly on January 1 and takes the deduction. We then have to file our return through the mail, and attach copies of his divorce agreement, and he gets the exemption. I don't know if his ex is being penalized. Honestly, she doesn't seem to be since she continues to do it year after year.

My husband does eventually get the dependent claim that is rightfully his, but it takes a few months rather than a few days if his ex wasn't screwing around.