Brief answer: time signatures were not really standardized until the 20th century. Schubert used one of the many variations still seen in his time for this unusual meter, which is basically what we would write today as 4/2.
For some more detail, originally C was used in the late renaissance typically to indicate what we'd now think of as a 4/2 or 2/1 meter, with the beat typically felt on the semibreve ("whole note"). The C with a slash down the middle ("cut C") came to be used to indicate a faster tempo, or that the beat should be felt on a larger level, i.e., the breve ("double whole note"), hence the term alla breve. (We still use the term alla breve even though we rarely write breves in modern notation, and certainly almost never feel them as the primary beat.)
However, this all predates standard use of barlines. So how many minims or semibreves or breves you might think of as within one "unit" was a bit arbitrary. Once barlines started to become common in the 17th century, time signature use was very inconsistent. The C could still be used to represent a 4/2 meter, but it gradually came to more frequently indicate a 4/4 meter. It was partly these sorts of confusions that led fractional notation for time signatures to be adopted.
Cut C was a more complicated case, however. There was still a need in the 18th century to represent the so-called stile antico ("old style") that imitated Palestrina which was assumed to be in 4/2 time. While some styles of 18th-century music adopted the cut C to mean 2/2, old church style pieces interpreted the idea of the "longer note value having the beat" to mean that 4/4 was effectively lengthened to 4/2 measures. Cut C thus could represent both 2/2 and 4/2.
The potential source of confusion caused a variety of other symbols to be used for one or the other to create some clarification. One often sees simply "2" as a time signature to indicate 2/2 in this period. But 4/2 developed plenty of variants as well. For whatever reason, the fractional notation took a while to take hold for 4/2, and instead composers employed a wide variety of abbreviations.
For the traditional alla breve (4/2), cut C could simply be used (occasionally even just a plain C), or sometimes a backwards version of C or cut C. But then you had a bizarre revival of "cut circle," i.e., a circle with a vertical line through it (which in the renaissance represented a sort of fast triple meter) to mean 4/2. (See its use, for example, in the Credo to Bach's Mass in B minor.) Cut circle was perhaps interpreted as a sort of dual C both forwards and backwards, hence doubled in length.
Other composers and theorists recommended a C with a double vertical slash to indicate 4/2 alla breve. Even later, some composers also used a doubled C or doubled cut C indication for 4/2 as well. Hence what you see in Schubert here. All of these come out of the problem of whether alla breve should be interpreted as a 4/4 with a faster beat (i.e., 2/2) or a 4 beat measure with a longer note duration used as its beat (i.e., 4/2). Everyone knew that "cut C" meant alla breve, but the problem was what actual time signature that corresponded to.
Does it mean anything different from 4/2? Probably not, but it's difficult to say as practice was non-standardized, and time signatures could have regional variations and meanings. Time signatures until the 18th century also often were meant to convey tempo and stylistic indications, but those implications were gradually phased out in favor of Italian terms that we see today (e.g., "Allegro," "Adagio," etc.). A couple generations earlier, there might have been some significance to writing a time signature in a slightly different way (e.g., a 3/4 meter might sometimes be written as C3 or cut-C3 to indicate different tempos). And the use of "cut C" for alla breve in the 18th century for two different meters played off the fact that it could indicate two different styles (a fast 2/2 galant style or a more reserved stile antico 4/2 in old church style). The stylistic implications were part of the reason it took so long for the ambiguity in this symbol to get resolved.
But Schubert's explicitly doubled symbol has none of that ambiguity, and was probably just one of the many variants seen in his time to indicate what was an increasingly unusual meter.
For more detail about this whole transition, the confusion about alla breve, etc., I might recommend Roger Grant's Beating Time & Measuring Music in the Early Modern Era (2014). (For what it's worth, Grant thinks Schubert here was using a variant of the double-slashed C that was used by some in the late 18th and early 19th century. For example, some copies and editions of Bach's E major fugue from the WTC appear with that double-slashed C notation.)
