Must one repeat the Shahadatain if they are assailed by Satanic whispers concerning religious matters?

The cure for obsessive thoughts is to ignore them and pay them no mind. There is no need to utter the Two Testimonies if you have not exited Islam. Whenever an obsessive thought comes, say: "I believe in Allah." As for possession by jinn or the devil, it is treated with ruqya (incantations) and incantations authentically reported from the Prophet, peace and blessings be upon him. One can recite ruqya upon oneself using Al-Fatihah, Ayatul-Kursi, and the Mu'awwidhat (Surahs Al-Ikhlas, Al-Falaq, and An-Nas). With persistence, Allah will remove what the afflicted person is experiencing.