In Series 2 Episode 3 (Thanks for the Memory) Lister gives Rimmer 9 months of his memory as a 'death-day' present.

When this is finally revealed to Rimmer, Rimmer starts realising the obvious incongruencies cause by these memories and says the line "That's why I had my appendix removed, twice.".

This implies that in the 9 month memory Lister gave Rimmer, Lister had his appendix removed.

But then later in series 6 episode 2 (Legion), Legion removes Lister's appendix, telling him he was on the verge of peritinitous.

Thus Lister had his appendix removed twice.

--Camo