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Pretty similar are:
Platydoris cinerobranchiata which differs by uniformly bluish-grey gills and by brownish-green rhinophores and
Platydoris cruenta which differs by its hair-like brown lines on the mantle.
Platydoris scabra which lacks red patches on the mantle and differs by brownish rhinophores.
More information on Platydoris formosa are on Bill Rudman's Sea-Slug Forum!