That's an IRS reject and it means there is a duplicate dependent SSN.
I don't know whether that means you have entered the same SSN for 2 of your dependents, or if someone else has already filed a return with a dependent having the same SSN as one of yours.
The text from the IRS electronic filing business rules:
Each 'DependentSSN' in 'DependentDetail' ('QualifyingChildIdentifyingNum' in
'QualifyingChildInfoSSPRGrp' for 1040-SS(PR)) in the return must be unique among all dependent
SSNs.
