Gemmorah in Yoma 87B and Rambam Teshuva here rule to say Viduy over and over on Yom Kippur.
ת"ר: מצות וידוי ערב יוה"כ עם חשכה
The Sages taught: The main mitzva of confession is [just once] on Yom Kippur eve when darkness falls.
However, we repeat it twice every Teffila on Y"K:
אבל אמרו חכמים יתודה קודם שיאכל וישתה שמא תטרף דעתו בסעודה ואע"פ שהתודה קודם שאכל ושתה מתודה לאחר שיאכל וישתה שמא אירע דבר קלקלה בסעודה ואף על פי שהתודה ערבית יתודה שחרית שחרית יתודה במוסף במוסף יתודה במנחה במנחה יתודה בנעילה
But the Sages said: One should also confess on Yom Kippur eve before he eats and drinks at his last meal before the fast lest he becomes confused at the meal, due to the abundance of food and drink, and be unable to confess afterward. And although one confessed before he ate and drank, he confesses again after he eats and drinks, as perhaps he committed some sin during the meal itself. And although one confessed during the evening prayer on the night of Yom Kippur, he should confess again during the morning prayer. Likewise, although one confessed during the morning prayer, he should still confess during the additional prayer. Similarly, although one confessed during the additional prayer, he should also confess during the afternoon prayer; and although one confessed during the afternoon prayer, he should confess again during the closing prayer [ne’ila].
This seems contradictory to me: if the Mitzvah is related to Y"K - let's say it once (as T"R) in Ma'ariv, if we suspect one can transgress additional prohibitions - lets add it to every Teffilah all year round.
Why Viduy is needed so many times?