Apparently, there's a genderswaped version of Nagato Yuki tagged as Nagato Yuuki. As the longer "double U" phonetic tag is very common on Danbooru, the latter tag can be easily mistaken for the former.
Based on the wiki page of the "Yuuki", it was tagged such because the only difference of their name being that "Yuuki" uses a phonetically longer spelling compared to Yuki. Though given the status as a fanmade character, I don't know how we should disambiguate between the two tags?