overtly, NEVER! we're all one big happy trade confederation.
covertly, all the time. "that planet is part of my duchy!" "no, MY duchy!" "MINE!" bam biff boom boff ... all behind the scenes of course. if you need a justification for piracy, there it is. not to mention "planetary navies", trade wars, missing ships, factions, coups, "uprisings", "revolutions", mercenaries, you name it.
could even be a matter of imperial policy, keep the nobility fighting each other instead of coming after the emperor.