IRS e-file reject - dependent previously claimed?

[Kea]

IRS rejected return for dependent SSN already used. The ex is the custodial parent and did not claim child (I saw page one of her return). But ex is custodial parent and apparently used dependent care and EIC (per "client"*). Could someone else claiming the credits cause the e-file to bounce even if they didn't claim dependency?

IRS computer hiccup or maybe someone else claimed the child?

* FWIW, this is a friend of a relative who used me one year and used Turbo Tax since. He was just asking me what the error meant. In reviewing his return I see a couple of errors (i.e. no depreciation on a rental property). So he might become a client again.

[Kea]

He sent over specific error code / message. IRS flagged 2 people claiming same SSN on 2 different 2441s.

[David1980]

Is the non-custodial parent trying to claim the SSN on 2441?

[Kea]

It seems both parents claimed 2441.

"Dad" had said previosly that "Mom" was custodial parent. That was based on divorce decree. But daughter lives with Dad 4 nights per week and with Mom 3.

Dad said TT didn't ask questions about depreciating his rental house and appliances (at least to his understanding). So he'll be returning as a client. We still won't be able to claim 2441 unless Mom ammends. He doesn't want to cause problems w/ ex by paper filing with documentation.