There is an obligation to confess one’s sins on Yom Kippur. The Sages state that confessing once does not suffice. One must recite the confession several times, in each of the prayers of Yom Kippur.

The Sages taught: The mitzva of confession is just before Yom Kippur when darkness falls. But the Sages said: One should confess before Yom Kippur before he eats and drinks the final meal before the fast, lest he become confused at the meal due to excessive eating and drinking. Even though one confessed before he ate and drank, he confesses again after he eats and drinks, as perhaps something improper transpired during the meal.

Even though one confessed during the evening prayer, he must confess in the morning prayer. And even though he confessed in the morning prayer, he must confess in the additional prayer. And even though he confessed in the additional prayer, he must confess during the afternoon prayer. And even though he confessed in the afternoon prayer, he must confess in the closing prayer. (Yoma 87b)