Your qualifying child's SSN cannot be used more than once in a tax return OR on another tax return filed and accepted for the same tax year.
Fix #1: Sign into your PriorTax account and make sure that you entered the correct information for your qualifying child. You can make any necessary updates on your rejection status page then resubmit for filing.
Fix #2: If you know who used your qualifying child's SSN on their tax return, you can contact them directly. They can amend their tax return and refile it without claiming your child as a dependent. Then you can resubmit your return with the child claimed on your tax return.
Fix #3: If you are unsure about who claimed your child on their tax return, you should contact the IRS for steps on how to proceed with filing your tax return. You may need to paperfile your return this tax year if you choose to claim the child. You will need to provide the IRS with sufficient documentation as to why you are eligible to claim the child on your tax return.