Accidentals always overrule the key signature. So, seeing an F with a sharp sign in a key signature when that is already sharped does not mean double sharp. It means F(natural) sharp. As pointed out, reminders are sometimes prudent, especially if the note was natural-ed in a recent measure. If it was natural-ed earlier in the same measure, then the accidental is required, otherwise the natural would still apply without the sharp accidental. Since double sharps and flats never occur in key signatures, they will always be explicitly shown when they're needed.