EIC Can not be used more than once?

I have been trying to file my taxes and I keep getting the message that EIC Social security number can not be used more than once.  My daughter lives with me and I have residential custody, and her mother and I switch every year who claims the Child deduction, but I still should be qualified for the EIC.  It appears her mother has filed for this, what are my options to contest this and file so I get the EIC credit?