According to this site, a Double Sharp can be notated as ♯♯. But is there any score that contains this notation?
You can do it, but most likely you shouldn’t. 𝄪 is the standard character used for this sort of thing, both in engraved scores and handwritten scores.
Historically though we need to consider that the double sharp is a fairly new symbol, coming up after 1700. If you are interested in the history consider page 94 here https://zenodo.org/record/2387750
If you consider some scores e.g. in Bach’s WTC you will find that no sign is used for 𝄪, but rather an additional sharp is placed in front of the note (adding one sharp to the key signature).
Furthermore this notation is viable if you are limited to a musical typesetting system that does not offer these characters.
In any case the website you linked should make it clear that 𝄪 is the preferred way to do it since the 1750s.
The website is incorrect. "##" is not used, nor has it been used, in musical scores.
- Elaine Gould makes no mention of the symbol in Behind Bars, and uses only "x" for "double sharp".
- The Oxford Companion to Music (ed. Alison Latham, 2002) only mentions the "x" symbol in its "Double sharp" entry (p. 377). The "Accidentals" entry (p. 4) does show two other symbols used historically in the early evolution of double sharp notation, but both are long obsolete.
- The Norton/Grove Concise Encyclopedia of Music (ed. Stanley Sadie, 1994) uses only the "x" symbol in its entries for "Accidental" (p. 4) and "Double sharp" (p. 230).
One does sometimes see "##" used in online posts. For example, it appears on this site: