By redundant I mean that it wouldn't make much of a difference if it didn't have the ability.
And just to clarify I'm not talking about abilities that Pokémon can use but can't do much with.
As for me I have a few.
Rotom-air: It's already flying type, so it's immune to ground type moves by default.
Rhyperior: (Lighting road) In double battles it might be useful, but in single battles it does literally nothing. The boost doesn't work for him since he's immune to electric by default
Stunfisk: (Limber) In the generation it was introduced in it might have had a point, but now electric-types are immune by default. So it does nothing.