I'm guessing somehow they merged the two Rimmers. Mostly though we're expected to just think they're the same character no matter what. Incidentally, if Rimmer's dad wasn't lying (and I think he coulda been), the gardener musta had more fun with the wife, as one of Arnold's brothers had also been played by Chris Barrie. And HE was a super successful guy too.
Decent episode. Overall a good season. It fits - I think that if somehow Red Dwarf had been on for 25 years without a break, its quality would be about what we saw.