Is it obligatory for the congregant to follow the imam who says one taslim after the tashahhud and before the prostration of forgetfulness, and then says two taslims after the prostration and the final tashahhud, or is this a mistake that necessitates leaving him?

The Hanafis are of the view that the prostration of forgetfulness (sujud al-sahw) occurs after a single salutation to the right. If one prostrates before the salutation, it is permissible but disliked. If one gives two salutations, the prostration of forgetfulness is waived. It is obligatory to repeat the tashahhud after the prostration of forgetfulness. The follower must follow their Hanafi imam. The majority of jurists deem it permissible to suffice with a single salutation, and this is what is followed in this madhhab. The follower must give the salutation with the imam and then perform the prostration of forgetfulness.