On Yom Kippur, the Kohen Gadol would say Achas, Achas v’Achas, Achas u’Shtayim until ‘Achas va’Sheva’

The Imrei Emes explains that the most precious moment for Hashem is when one makes a strong resolve to improve his ways. Afterwards, he may have ups and downs, but it is this first firm resolve which is so precious to Hashem.

That is the reason the Kohen Gadol continually repeats achas (one) representing that first moment that he accepted upon himself to improve his ways.