Yes. This just happened to me twice today.  In both cases, the parents are married so nobody is competing to claim the dependent, the children are under 8 years old, there are no typos in the names or Social Security numbers for the children.  And yet their returns were rejected and I got notices that "A dependent on your return has already been claimed on another tax return for this year."  I suspect a software glitch, as there is NO reason to think one of these families would be the victim of identity theft, let alone two in one day.