In Musaf of Yom Kippur, we say a prayer (page 574 in the Artscroll Machzor) which begins “umerov avoneinu ein lanu:” and then it goes through a double acrostic of things we no longer have, beginning with ishim and asham (aleph). Then badim and b’lulot (bet) etc.
But when it gets to vav, the two options are vidui and par chatat! Why no double Vav? I hesitate to think that the composer couldn't find another word or phrase to fit in. I can see that those two ideas are related but for every other letter, two words/phrases which begin with the same letter are listed. Why would it be different for this one letter?