My son asked me to settle an argument he was having with some friends earlier - is the plural of grapefruit the same, or can it take an s? My instinct was to say the plural was the same, and, indeed, that was what the one-volume Oxford Dictionary of English says. Collins, however, gives both variants.
Perhaps it's a word like fish, where the usual plural is the same, but you can also say fishes. AskOxford.com says fishes is an older plural, and says it tends to be referring to different types of fish. The word fruit has two plurals, too; 'I like all fruit' and 'I like all fruits' both seem acceptable to me. Like fish/fishes fruit seems to lump everything together and consider it the same, whilst fruits suggests different kinds of fruit. On the other hand,I wouldn't say 'I ate four fruits today', even if they were all different, I would say 'I ate four pieces of fruit'.