In Slim/Music/Info.pm it appears that DISC and DISCC are declared twice. Since
these values are later treated as hash keys, I'm guessing that the first
declaration gets lost.

my @infoCacheItems = (
...
        'DISC', # album number in a set 
        'DISCC', # number of albums in a set
...
        'DISC', # disc number
        'DISCC', # disc count
);