I've noticed Daeron's Runes seems to find a place in many of the top tier decks... is it worth buying another copy of the AP it came in?

Also, while we're on the subject... are there any AP's that should be bought in duplicate for the great cards they yield?

If you don't want to shell out the extra cash for the APs, you can go on to the team covenant website, where I think they sell singles of some of the more useful cards. It really just comes down to preference, and more importantly, your budget. I think there is added convenience of not having to tear apart player and encounter decks, but, at least for me, it isn't worth the extra money.

Maybe I've got it wrong, but isn't 3 copies of a card the maximum of what you're allowed to keep in a single deck? Otherwise I would stuff mine full with Gandalfs and Unexpected Courage.

You're right. He probably wants to include Daeron's Runes in multiple decks.

I had a friend who bought two copies of every lotr product in existence because he was regularly hosting 3-4 player games and it is practically necessary at that point. Once he had converted half the people playing with him (myself included) to get their own sets he stopped that route.

A lot of solo players also play two handed to really enjoy the game at the optimal level. Another reason to buy extras would be the aforementioned idea of leaving multiple decks constructed at a time for convenience. Personally this is the least appealing reason to pick up dupes but still worth it to some people. Personally I just write down the cards I'm stealing on a piece of paper and leave it with the deck not currently in use.

When do dupes come in handy the most? Honestly... even if you only rarely host 2 player games, having dupes of this card, some of the cheap green dwarves, and other similar card types really expands your options. It is totally worth it in that case