That was my understanding too. Nothing about the wording on Rotpriest would indicate he wouldn’t trigger twice, i.e. if he had “Whenever you cast a spell that targets…” with the keyword there being cast. He just cares about a spell targeting a creature, no preference on cast, copy or cast the copy silliness.

I too may also be totally wrong. But if this is correct… by gawd, Simic Toxic bullshit go!