In the key of G Major, what does an F double sharp become (keeping in mind the key signature has F# in it)?
I am no expert, however I have seen similar calculations, that would turn this into an, 'Altered Scale' ... of the 'Ab 'Harmonic Scale. I have no criteria, to back up my 'guess !
Sometimes a sharp is repeated just as a reminder. If you come across a piece in G major and see an f with a sharp it is just a reminder so in order to make F sharp, sharp (which is g) you would need a double sharp. in short the answer to your question is g. though im not sure of the context you are seeing it in.