Or HaChaim on Deuteronomy 17:6/אור החיים על דברים י״ז:ו׳

1 יומת המת, "the condemned person will be executed." The reason the Torah repeats the reference to the condemned being dead is to alert us that if no witnesses could be found or that the warning was not legally valid for some reason, the fact that the terrestrial court could not execute the sinner does not make him any less guilty of death in the eyes of G'd. The word יומת applies when the legal proceedings here on earth could be completed.