This whole verse is about the repetitive nature of time, especially when working a 9-5 and the days start to repeat.
Deja Vu is a French saying for living the same situation over and over again, and c'est la vie is a French saying for “such is life.” When Deja Vu turns to c'est la vie is when you grow so accustomed to the repeating days you accept it for what it is.