If I think I filed the the same 2018 return twice, how can I fix this?

No problem IRS will automatically reject a second attempt at filing an accepted return.

There is nothing to fix or worry about -- if you already e-filed and the return was accepted, then any second attempt is rejected.  When it could be a problem is if you e-filed and then mailed in the same return, or vice versa--because that could confuse the IRS.

To check your e-file status 

https://turbotax.intuit.com/tax-tools/efile-status-lookup/