Normally you reach a decision on the kind of redundancy and technological diversity by looking at certain risks and then decide if you only need one, two of the same, or two or more of differently implemented/built devices using different methods of operation.
For (made up example, I don't know anything in particular about aviation, but know the general approach from working with controls for nuclear powerplants) navigation one might conclude that GPS and old-fashioned UHF radio navigation is technologically different enough that an event taking down one means of navigation (say, jamming of GPS at 1.5 GHz, spread spectrum, digital, ...) will not affect UHF radio (at analog modulation on a few hundred MHz). So one decides that you have GPS and UHF, displayed on different monitors/indicators, powered by separate power-busses in the airplane.
For paper documentation, they'll probably claim: Bring two binders with the procedures to the plane, store at different sides of the cockpit.
For iPads one might have decided that they must not be connected to any network while on the plane, so that data cannot be deleted from the pilot's and first officer's devices. But have they considered the "common cause failure" of two iPads, not being connected to any network, not communicating with each other, with full batteries, probably with passed selfchecks (checksums for stored data on the flash)... to really fail at the same time because of an expired certificate?