It could also be if you signed up on the IRS non-filer tool to give the IRS your Direct Deposit Info for the Stimulus Check.  If you did then see

https://ttlc.intuit.com/community/tax-topics/help/stimulus-how-to-file-your-return-after-getting-rej...

If you're saying that your social security number has already been used to file (make sure you typed it in correctly) then you may well have been the victim of identity fraud. I believe you need to address it with an agent of the IRS directly to find out what steps you need to take from there.