A. You’re right. Even if someone had concluded their prayers earlier and later joined people praying, there are a few portions of prayer that you’d be required to say along with the congregation. Amen for Kaddish, the response to the Kedusha, the Shema, and the concluding Aleinu Prayer. It’s like if you’d be sitting in Shea (this was written in response to a Met fan, but the same applies across the board) and the Mets score a triple-run homer in the bottom of the ninth, and the whole stadium is roaring, cheering wildly on it’s feet – and you stay seated in silence. That reflects negatively on your feeling for the Mets. Same with these important segments in prayer which exclaim our essential Jewish beliefs, we ought not to be silent when the congregation says them, even if we said them already. Same goes for the Shema, Kedusha, Kaddish and Barchu. Historical Notes: It is said that Joshua wrote this prayer, making it one of our oldest, and it was sung by Jews burning in auto-da-fes during the Spanish Inquisition.