וְשָׁמְר֥וּ בְנֵֽי־יִשְׂרָאֵ֖ל אֶת־הַשַּׁבָּ֑ת לַעֲשׂ֧וֹת אֶת־הַשַּׁבָּ֛ת לְדֹרֹתָ֖ם בְּרִ֥ית עוֹלָֽם׃
און די קינדער פֿון ישׂראל זאָלן היטן שבת, צו האַלטן שבת אױף זײערע דוֹר-דוֹרות; אַן אײביקער בונד.
Chizkuni
ושמרו בני ישראל, “The Children of Israel shall observe, etc,;” they shall have in mind the next Sabbath during the six working days, looking forward to it, and to prepare for it in order not to need to desecrate it when it comes. This expression was mentioned earlier in v. 14; it is repeated here on account of it having to match the expression ברית עולם, “an everlasting covenant.”
Sforno
ושמרו במי ישראל את השבת, in this life, לעשות את השבת, on a day that is totally Shabbat. [the afterlife. Ed]
Or HaChaim
ושמרו בני ישראל את השבת, "The children of Israel shall observe the Sabbath, etc." Why did the Torah add another "observance?" Besides, why does the Torah use the verb לעשות, "to do," which expresses an activity when the meaning seems to be that the children of Israel are to observe the Sabbath by "abstaining" from a number of activities? Our sages have pursued their own path in explaining this wording.