Komentaryo sa Exodo 31:14: Rashi, Chizkuni, Sforno & Or HaChaim

מות יומת SHALL SURELY BE PUT TO DEATH by the judges if there are witnesses testifying to the desecration and if the warning required by law had been given to the culprit immediately before he committed the offence.

ושמרתם את השבת, “you must observe the Sabbath;” the reason that this has been repeated is to warn that anyone desecrating the Sabbath purposely and publicly, will be executed. If he does so secretly, he will be cut off from membership in the Jewish people and its eternal future by an act of G-d. Seeing that the penalty has already been spelled out, the warning not to become guilty of this sin and its penalty has to be spelled out also; hence the Torah writes not only: מחלליה מות יומת, but also: כל העושה בו מלאכה מות יומת, “anyone performing forbidden activities on it will be executed.” It does not matter whether such forbidden activity is carried out by day or by night. This is made clear by the repetition of ויום השביעי קודש, “the entire seventh day is holy.”

ושמרתם את השבת, there is yet another reason why the Sabbath regulations must not be overridden for the sake of completing the Tabernacle sooner;

ונכרתה SHALL BE CUT OFF [FROM AMONG HIS PEOPLE] by God, if no such warning had been given (Mekhilta).