If I claimed the "Credit for other dependents" for my stepson, can his mother claim him as a "Qualifying Dependent" and get other credits?

[HardLuckAndy]

I was merely following along the tax software and apparently I unintentionally claimed my stepson as a "Credit for other dependents." All of the qualifications are legit. However, I moved out on December 16, 2019 and though we are still legally married, we are in the process of getting divorced and her attorney is claiming foul and that my ex-wife cannot claim her own son as a dependent. I did not intentionally do this, but before I pay increased legal fees for a tax attorney, if I already filed my 2019 tax return and my ex-wife did not, can she still claim her son for the remaining tax credits if she lists him as a Qualifying Dependent? Further, is there a way I can check to see if I even received anything? The box next to the child's name is "QUALIFIES FOR Credit for other dependents." Is there a chance I didn't claim him? Schedule A as attached to my tax return from the software does not specifically say anything about this credit. On my Stimulus Check, I did NOT receive $500, if this is any indication.

[Hal_Al]

Q. Can she still claim her son for the remaining tax credits if she lists him as a Qualifying Dependent? 

A. No. Although there is a special rule in the case of divorced or separated  parents, allowing the splitting of the tax benefits,  this "splitting"is not available to parents who lived together at any time during the last 6 months of the year. Then only one of you can claim the child for any tax reasons. 

Although either parent can claim the child, even a step parent,  if there is no agreement, on who will claim the child, the  parent the child lived with the most gets first choice.

The "Credit for other dependents" is (up to) $500, if you have a tax liability.  It is not on Schedule A, but is shown on line 13a of form 1040. 

Only children eligible for the Child Tax credit get you the $500 stimulus

[Critter]

She can claim the child however she must mail in the return  and both of you have to file using the Married Filing Separately status.

Then you must amend your return to remove the dependent.