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Does Warplightning count as the signature spell for Warplocks, and can an army have more than one Warplock who knows Warplightning?

Any spell you can swap out to as a "default" is a signature. If it gives you the option to remove a randomly generated spell for the result (or, in older books, remove a randomly generated if said spell isn't rolled with the other dice), then it should count as a "signature" and be duplicatable. Was Warp Lightning the Skaven spell for Skyre that you can swap out for?

Warlock engineers may always choose Warp Lightning, and as such you can have duplicates of it in the same army.