What is the ruling concerning the action of a congregant who performed a second prostration with the rest of the congregants, while the Imam was in his third prostration, and what is the ideal action if this occurs repeatedly?

If the Imam prostrates a third prostration out of ignorance, forgetfulness, or absent-mindedness, his prayer is valid, and he must perform the prostration of forgetfulness (Sujud as-Sahw). If he does not perform it, there is no blame upon him. As for the followers who performed two prostrations, their prayer is valid. However, whoever among the followers knows that his Imam will perform a third prostration, it is not permissible for him to follow him in it. Rather, he should perform his second prostration for himself and then follow the Imam in the Tashahhud. If he follows him deliberately and knowingly, his prayer becomes invalid. If he follows him out of ignorance or forgetfulness, his prayer is valid.