Why do we repeat the Amidah? is it ok to omit repetitions of some Amidahs?
The repetition of the amidah was enacted to fulfill the obligation of prayer for those who do not know the prayer themselves. They fulfill their obligation by hearing the prayer of the chazan (see Beis Yosef, Orach Chaim 124).
In addition, the repetition is the time when Kedushah is recited.
The Gemara (Berachos 32b) finds an allusion for repeating the Amidah from the double expression of the verse, “Hope to Hashem; strengthen your heart, and hope to Hashem.” The prayer is repeated in Shacharis and Minchah (and Mussaf), and not Maariv.