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Duallies have a narrower frame rear section to accomodate the inner rear wheel and most flareside pickups came on the standard, non-dually frame so they are wider.
Having said that, I believe you can adapt a standard flareside bed to work but you will have to fabricate custom wider rear fenders to cover dual rear wheels.
Also FWIW, a stepside bed is what Brand X call their flareside beds. They also call their style side beds "fleetside".
FBp