Drive maxed out


What should be a little over 100gb backup has somehow completely filled my external 1TB drive.
Another oddity I see is that in File Explorer, my external drive is displayed twice, as if duplicated.

Shouldn’t the backup just ignore anything it has already backed up and is unchanged?

What are the steps to recover from this?

Did you connect My Passport device on another computer for this concern?

No, always same laptop but occasionally disconnected during travel,etc.