Getting a "Duplicate Social Security Number" error when submitting dependent child's taxes

My wife and I filed jointly and listed our 17-year-old son as a dependent.  After successfully submitting our return, I attempted to file a separate return on behalf of our son since he worked part time last year (earning about $7500) and is due a refund from federal and state taxes.  I listed that he was being claimed on our return and filled everything out.  After paying for the direct deposit fees, I clicked on the "Transmit Returns Now" button and got the following error message:

"Duplicate Social Security Number.  A tax return with the same Social Security number has already been submitted - in other words, it appears you're trying to e-file the same return twice.  If you need to change this return, you'll need to file an amended return on paper, by mail.  If you'd like to e-file your state return using TurboTax, it will need to be filed along with your federal return.  You may also e-file your federal return now, then submit your state return later via TurboTax, by mail, or by visiting your state's website for alternative e-filing options."

 

Any idea what is going on and what can be done?  Did I need to purchase a separate copy of TurboTax in order to file his return?