It's done in purpose. There are also a bunch of similarly "duplicated" entries in c64.c. The idea is to have an entry for each different model sold (see also many Sharp Pocket Computers or Sega SC3000/SC3000h or Dina's different Coleco clones), possibly with different artworks (e.g. if we start to implement clickable keyboard we will need good scan of the different PET2001 'chicklet' and full stroke keyboards).
Will the different duplicate Master Systems be added? I remember a driver for the Master System III Compact (the first system I owned), but it was removed =(.