Are you sure someone else filed a return using your SSN or did you use the non filer portal to register for the stimulus payment. If you used the non filer portal, you actually efiled a return with 0 income on it for yourself. Filers were not supposed to use the non filer portal.
If that is what happened, you should have filed an amended return form 1040X or written "Amended Return" across the top of the "real" return you mailed before you mailed it.
The IRS was not processing mailed returns for awhile although it is reported that then may have started back up earlier this week. In normal times it takes 16 weeks for an amended return to be processed.
All you can do is continue tracking your refund and stimulus on the IRS portals:
Stimulus payment:
Get My Payment | Internal Revenue Service
Refund:
https://www.irs.gov/refunds
or https://www.irs.gov/filing/wheres-my-amended-return
