In some cases, an attempt to claim a child as a dependent when another person has already claimed him results in rejection of the electronic submission, according to About.com. If the IRS accepts the return or the second parent mails a return claiming the same child as a dependent, the IRS is likely to require an audit over dependents.
Unless parents are married filing joint returns, the IRS only allows one parent to claim a child as a dependent. About.com reminds parents this law also applies to married couples filing separate returns. The return claiming the child as a dependent should include Form 8332 signed by the other parent, giving permission to claim the child as a dependent, according to TurboTax.
If the IRS doesn't accept a tax return because someone else has claimed the child as a dependent, it doesn't allow electronic submission a second time. About.com recommends that a parent with the right to claim a child complete the paper form and mail it. The IRS audit determines which parent is entitled to the exemption.
TurboTax recommends that if a parent discovers he has claimed a dependent in error, he should file an amended tax return. The IRS allows such amendments within three years of filing the return. The IRS imposes fees in such cases in addition to requiring payment of the additional taxes owed; however, if a parent convinces the IRS claiming the child was a mistake, the IRS waives the penalties.