IRS Rejected Return: Son's SSN Shows As Already Filed - How To Fix Duplicate SSN Error

Filed my taxes and the IRS rejected it because of my sons social security number. I know its the right number because I'm looking at his card. What steps do I need to take to fix this and amend my return? Really need this refund and dont want to mess it up. Just got this message from the tax filing software saying "Your transmission didn't go through. We could not e-file your return for the following reason: 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." This doesn't make any sense because I'm absolutely positive I entered my son's SSN correctly - I'm literally looking at his social security card right now. I haven't filed twice, this is my first attempt for this year. Is it possible someone else claimed my son as a dependent? Or maybe someone used his SSN fraudulently? What do I do now - should I follow the instruction to file an amended return on paper by mail? Or should I contact the IRS directly first? I'm really counting on this refund and don't want to delay it further by taking the wrong steps. When I click "Retry" it just gives me the same error message. Do I need to get a new SSN for my son? This is really frustrating because I need this money soon.