Almost every certificated aircraft has two spark plugs per cylinder. That's for more efficient ignition, and it serves as basic safety redundancy. If one spark plug fails the cylinder keeps firing.
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Whether or not an airplane has power outlets at every seat depends on a few factors such as the airline, the type of aircraft, and the cabin class. In general, many modern long-haul planes have power outlets at every seat in the premium cabins (first and business class).
A twinjet or twin-engine jet is a jet aircraft powered by two engines. A twinjet is able to fly well enough to land with a single working engine, making it safer than a single-engine aircraft in the event of failure of an engine. Fuel efficiency of a twinjet is better than that of aircraft with more engines.