To prevent fraudulent returns, the IRS only accepts a SSN once in their e-file system.
In order to finish with your taxes today, you should print, sign, and mail in your return instead of e-filing. Here is a link with instructions on how to do that: https://ttlc.intuit.com/replies/3401713
If you understand why your SSN may already be in the system (because a parent may have claimed you as a dependent perhaps?) then you can stop there, if don't know why you may have gotten the error, this link may help you further with more information... https://ttlc.intuit.com/replies/3302450