What is the ruling on someone who sits for the tashahhud and doubts whether it is the first or second tashahhud?

Whoever doubts the number of rak'ahs in prayer should, according to the majority of scholars, base their action on the lesser number and perform two prostrations before the taslim, based on the hadith of Abu Sa'id. And whoever doubts whether it is the first or last tashahhud should base their action on it being the first tashahhud and perform two prostrations before the taslim.

Some scholars hold that the worshipper should strive to ascertain the correct number and act upon what they predominantly believe to be true, and then perform two prostrations after the taslim. This is the choice of Shaykh al-Islam Ibn Taymiyyah and Shaykh Al-Uthaymeen. So, if one predominantly believes one of the two possibilities to be true, they should act upon it, base their prayer on it, and perform two prostrations after the taslim. If nothing becomes preponderant in their mind, they should take the lesser number, base their prayer on it, and prostrate before the taslim.

There is no difference between the Imam, the person praying alone, and the congregant in basing their action on predominant belief, according to those who hold the view of taharri (striving to ascertain the truth). This is contrary to those who believe that the Imam acts on his predominant belief, while the congregant and the person praying alone act on certainty (the lesser number).