1) you used the IRS nonfiler to get the stimulus. From the IRS standpoint this constitutes filing your 2019 return
2) someone put the wrong SSN on their return
3) you're are a victim of identity theft.
4) somehow you did actually submit your return
the irs phone lines are closed so you could possibly see if a return was filed in your name by going to IRS website, registering and viewing a transcript of your return.
https://www.irs.gov/individuals/get-transcript
if no transcript then number 3 is less likely but it would depend on when they filed becuase the transcript is not immediately available even if efiled.
if there is a trascript does it look like your info then 4 is likely so don't refile
otherwise paper file you can not efile.