In MuseScore version 3.6.2 I see it being F##5 indeed, but the next note in line is also F##, but looking into how it looks like in Piano visualiser that first one must be a mistake.
1 Answer
It should be F#5; the score in the Question contains an error.
Here is the same measure in the Busoni edition of the piece. The note has a # sign in front of it.
(Image source: IMSLP, Grandes études de Paganini, S.141)
And again, from the Gárdonyi edition (1971)
(Image source: IMSLP, Grandes études de Paganini, S.141)
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2@ВалерийЗаподовников the alternative conclusion, that there are supposed to be 12 perfect octaves with one diminished octave in their midst, is not tenable.– phoogFeb 5 at 8:13
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It's indeed a logical conclusion - what else! But could there have been a cautionary natural sign included (cancelling one of the sharps before) . Almost the whole piece is filled with accidentals, let's make it a full set...– TimFeb 5 at 9:17
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I would only note that in the Gárdonyi (Bärenreiter) urtext the correction doesn't even merit a comment in the Critical Notes at the back of the volume. (Where editions still differ is in the quasi cadenza, m.77.) Feb 5 at 15:43