1)
Why does the Torah use the double expression "Im Shacho'ach Tishkach"?
1.
Rashi (in 11:13): To warn Yisrael that once one begins to forget, one goes on to forget everything'. 1
2.
Mechilta (in Beshalach): If you forgret one Mitzvah, you will end up forgetting many Mitzvos. 2
1
Rashi: As it states in Megilas Chasidim (or Megilas S'tarim - [It is also a Sifri]) 'If you will abandon Me for one day, I will abandon you for two!' (See Sifsei Chachamim there. In other words, when you forget Hashem, He forgets you
2)
Why does the Torah use the double expression "Ki Avod Toveidun"?