Re Pendle Hill, apparently there's a "Torpenhow Hill" which translates as "Hill-hill-hill Hill".

No-one's yet mentioned "MIDI interface".

I'd quibble with some of the others. "Hot-water heater" isn't necessarily a tautology, because you might not necessarily be heating the water until it's hot. You could conceivably have a "tepid-water heater" and a "slightly-above-freezing water heater" (a car block heater, for example).

And "repeat that again" is only a tautology if you haven't previously repeated it.

Graham.