Must I repeat the prayer if I prayed in a mosque that contains graves and shrines, and has a dome over it, and its Imam was a Sufi? And am I sinful for knowing this?

Prayer is not permissible in a mosque built over a grave or in which a grave has been placed, due to the narrations cursing those who take graves as places of worship. Prayer is not valid in such a mosque, and therefore, the prayer must be repeated.

Regarding praying behind a Sufi, there is a detailed explanation: If he is involved in a polytheistic innovation, such as invoking other than Allah, then prayer behind him is not valid. However, if his innovation is not polytheistic, then prayer behind him is valid.