how to determine if my dependent has been accidentally claimed

[lemonlives]

I am continuously getting rejected for a problem with my dependent I'm claiming I have

2 kids but one of them have for a fact has been claimed by the dad and the other one is supposed to be claimed by me. when the dad did his taxes he put both kids but only claimed one as dependent and the other one as non dependent. I did the exact same thing but the other way around claiming the kid he did not, it kept getting rejected. I am just confused I took off both kids on mine and and just put the one kid which then the return went down AND still got rejected so im in need of help im not understanding 

[LeticiaF1]

When your return gets rejected because one or more of the Qualifying Child SSNs on Schedule EIC has already been claimed on another return. If you have multiple children listed on your return, there'll be a number in brackets, [?], that identifies which one is causing this reject. 

For example: "[2]" will identify the second Qualifying Child on Schedule EIC. Make sure that the child's information has been entered accurately. Correct any misspellings, omissions, or transpositions. Make sure the child's SSN is entered correctly. If the Qualifying Child's information is correct, and you want to electronically file this return as it was originally submitted, an Identity Protection PIN will be required. 

How do I get my 6-digit IP PIN?