Numerous Repetitions
The mitzvah of Shabbos is repeated numerous times throughout the Torah. The Chafetz Chaim, several times in his sefarim, stresses that the Torah actually commands us regarding the mitzvah of Shabbos twelve times! From the constant reiterations, it is apparent just how holy and fundamental this mitzvah is.
(שם עולם פ' א. נדחי ישראל פ' לו. חפץ חיים עה"ת)
We find in many seforim that esides for confirming how important the mitzvah of Shabbos is, the numerous references to the commandments each convey or allude to another aspect of the holiness of Shabbos.