Or HaChaim on Genesis 2:16/אור החיים על בראשית ב׳:ט״ז

1 ויצו השם אלוקים על האדם. The Lord G'd commanded Adam. The Torah added the word לאמור, to say, and repeated the word for "to eat," by saying אכול תאכל. The Torah also repeated that violating the command would result in death by saying: i.e. מות תמות, instead of merely תמות. The reason is 1) that Adam should command Eve not to eat from the tree of knowledge either; b) that the penalty would apply to her just as much as to Adam himself (who had heard the commandment from G'd directly). The repetition of these words also constitutes both a warning and a spelling out of the penalty, something that halachah demands when capital punishment is involved.

3Another meaning of the repetition of the words מות תמות may be simply to give notice to Adam that should he eat from that tree he would cause his own mortality as well as the mortality of all those who would be descended from him in the future. This is, of course, what happened, and as a result we are all mortal until the day in the future when the pollution of the original serpent will have dissipated completely and G'd will banish death from earth (Isaiah 25,8).