Page "Accidental (music)" Paragraph 16

Note that in a few cases the accidental might change the note by more than a semitone: for example, if a G sharp is followed in the same measure by a G flat, the flat sign on the latter note means it will be two semitones lower than if no accidental were present.

Thus, the effect of the accidental has to be understood in relation to the " natural " meaning of the note's staff position.

For the sake of clarity, some composers put a natural in front of the accidental.

Thus, if in this example the composer actually wanted the note a semitone lower than G-natural, he might put first a natural sign to cancel the previous G-sharp, then the flat.

However, under most contexts, an F-sharp could be used instead.