I imported the same review file twice. Will my products now show every review two times?

Yes, if the first import actually worked. Each import adds every row as a new review, and there is no check for reviews that already exist.

To check, go to Marketing → Reviews and Ratings and click Import Reviews. Under Import history, each run shows its Rows and Imported counts. If both runs imported rows, you have duplicates.

To remove them, go back to Reviews and Ratings and click Delete on each extra review, then confirm.

Clear history only empties the import log. It does not remove any reviews.

Next time, look at the Import summary and View errors before uploading again.