<prompt> Human: Went to file taxes, asked ex for his sons SS she then informed him she already claimed him even though it was his year. They have joint custody so they typically alternate years. Can he file a claim too and then just let IRS sort it out? I dunno much about taxes honestly, and hes beyond mad, and Id like to offer some suggestions to him. He lives with me, so if theres any additional info needed I could get it.  <response>: He can claim the child, the IRS will catch it, and the childs school address will dictate who gets to claim the dependent. I am a former IRS tax examiner. 