On one of the Passage Through Mirkwood quest cards (3B, Don't Leave the Path), the text says you must defeat the Spawn of Ungoliant card to finish the quest. What if you have already defeated that card before? Do you have to dig it out of the discard pile, or wait until it comes up again? Or is the game over because you have already defeated Spawn of Ungoliant?

no you have to defeat it again. very annoying if it comes out twice

rich

If you have already defeated it, put it into play from the discard pile.

Do I put it into play immediately, or do I have to go through the process of exhausting the encounter deck, adding the discard pile into it, and going through the deck again?

You play it immediately from the discard pile to the staging area, and leave the rest of the discarded cards where they are.