I have joint custody of my daughter and pay child support to her mother. In 2012 her mother claimed her but had to give me half of her tax return per court documents. Thr documents state beginning 2013 I can claim my daughter for every year thereafter. I filed my taxes and they were rejected due to "someone" had already claimed my daughter. What can I do?

This has become more commonplace in the era of electronic filing. If one person files a return claiming the child, it blocks anyone else from filing electronically claiming the same dependent. So the answer is you have file by mail. Include a copy of the court order stating you have the right to claim your daughter and let the IRS sort it out.

Scott covers it all very well. It WILL take time, but the IRS will figure it out.