It seems to be unnecessary. Also, why do the specific Hatarat Nedarim on Erev Rosh Hashanah if we do it anyway just before Yom Kippur?
yeshiva.co was posed this question ("Hatarat Nedarim & Kol Nidrei") and responded:
1) The most common answer [...] is that Kol Nidrei refers to the vows of the community, while Hatarat Nedarim takes care of personal vows.
2) On another level, Kol Nidrei (according to one Nusach) frees us from future vows only if the condition is forgotten later, while Hatarat Nedarim is for all personal vows.
While this answer doesn't cite sources, dafyomi.co.il's "insights to Nedarim 23" cite Rishonim who also distinguish between the two:
The Meiri writes that "Kol Nidrei" does not serve to annul ordinary Nedarim. Rather, it serves to annul the Nedarim and Charamim made by the community, the Tzibur, as a whole. The removal of such Nedarim required neither the Hatarah of a Chacham or Beis Din nor Charatah, and thus it may be done through the recitation of "Kol Nidrei."
This sounds like reason #1 cited above.
The Nimukei Yosef explains that "Kol Nidrei" is not an act of Hataras Nedarim for either past Nedarim or future ones. Rather, it is a prayer to Hashem that He not punish us for the past Nedarim that we made and transgressed.
Worth checking out the link for other shitot quoted.