I have some question about "corrupt" , for example, Seductive Delusion(Legends,#35) .Can I target a unit have been corrupted? if I could target it , can the 1st unit deal damage to a 2nd unit? weither Yes or Not , pls describe your reason.

FAQ 1.5. p4:

If a card uses the word “then” the
preceding effect must have resolved
successfully before the effect following
the term “then” can be resolved.

I suppose a corrupted unit can be targeted, but since it cannot be corrupted again the corrupting effect cannot resolve, and therefore the "then" effect (i.e. the damage) cannot take resolve either and the action will fizzle.