Rashi on Mishpatim 23:31

English: ושתי is of the root שית or שות. The ת is dageshed, because it comes in place of two ת’s (ושת-תי) — one is necessary because no grammatical form of שות can be without the ת of the root, and the other is a servile letter (part of the suffix). עד הנהר UNTO THE RIVER — the Euphrates. וגרשתמו means AND THOU SHALT DRIVE THEM OUT.