Page "Accidental (music)" Paragraph 18

Double accidentals raise or lower the pitch of a note by two semitones, an innovation developed as early as 1615.

This applies to the written note, ignoring key signature.

An F with a double sharp applied raises it a whole step so it is enharmonically equivalent to a G. Usage varies on how to notate the situation in which a note with a double sharp is followed in the same measure by a note with a single sharp: some publications simply use the single accidental for the latter note, whereas others use a combination of a natural and a sharp, with the natural being understood to apply to only the second sharp.