This was explained by Rachel herself in the white hot room. There are indeed other Rachels that are the daughters of Scotts and Jeans across the multiverse but for some reason they aren't true alternates of Rachel herself. Canonically she is multiversally unique.
Wasn't there a Phoenix-wielding Rachel in the Secret Wars: X-Tinction Agenda miniseries? Though, it being an SW thing, I'm not sure how much it "counts."