Is one obligated to repeat the Fajr prayer if the Imam said the tasleem and the worshiper was not certain that he had recited the last tashahhud, with the predominant assumption being that he had not recited it?

Whoever doubts reciting the Tashahhud (definition) before the Tasleem, must perform it and then say the Tasleem, whether he is an Imam, a congregant, or praying alone. If he says the Tasleem before performing it, he must repeat the prayer, because the Tashahhud is a pillar, and doubting it before the Tasleem is like intentionally omitting it.