Q. Can you say Kol Nidrei if you were already mekabel (accepted) Yom Kippur. (As when saying Tefilas Zakah before)?
A. Remah (O.H. 619: 1) writes that Kol Nidrei should be said while it is still day, since at least according to some opinions it is an act of absolution by Beis Din of past oaths and promises, and should therefore not be said during Shabbos, Yom Tov or Yom Kippur.
Mishna Berura (5) writes that although we maintain that Kol Nidrei is a declaration for the future, meaning that forthcoming nedarim and oaths should not take effect, it has the appearance of an absolution. Matte Efraim and other Poskim also mention that when delayed it can be recited after the Shekia also. (Kaf Hachaim 619: 25, Piskei Teshuvos ibid: 2, and others)
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a