How do Strionic Resonator and Charmbreaker Devils interact?

Question

I'm working on a red instant and sorceries deck featuring Charmbreaker Devils. As I understand it, this ability:

At the beginning of your upkeep, return an instant or sorcery card at random from your graveyard to your hand.

is a triggered ability. So I should be able to use Strionic Resonator to copy it. How exactly does copying it work? Is there a chance that the same card in my graveyard is chosen twice, or will I always get two instants or sorceries back, as long as there are two to start with?

Answer 1

You will get two cards.

The following describes what happens, in order:

1. Charmbreaker Devils's ability triggers.
2. Charmbreaker Devils's ability is placed on the stack. [Stack: CD]
3. You activate Strionic Resonator's ability targeting Charmbreaker Devils's ability. [Stack: SR, CD]
4. Strionic Resonator's ability resolves.
   1. Make a copy of Charmbreaker Devils's ability. [Stack: CD#2, CD#1]
5. Charmbreaker Devils's ability #2 resolves.
   1. Return an instant or sorcery card at random from your graveyard to your hand. [Stack: CD#1]
6. Charmbreaker Devils's ability #1 resolves.
   1. Return an instant or sorcery card at random from your graveyard to your hand. [Stack: -empty-]

Whichever card was returned to your hand by #2 (step 5) will not be around when #1 resolves (step 6). You cannot obtain the same card from both (unless you cast it between step 5 and 6).

Answer 2

You will always get two cards. Here's why:

First: Charmbreaker Devils will put its triggered ability on the stack.

Second: Before that resolves, you may activate Strionic Resonator. One of its rulings states:

7/1/2013: Strionic Resonator targets a triggered ability that has triggered and is on the stack.

So you copy the triggered ability currently on the stack, and put the copy on the stack above the original.

Third: You then do nothing, and let these resolve.

Fourth: The copy resolves first. You return a random instant or sorcery card from your graveyard to your hand.

Fifth: The original resolves. You return a random instant or sorcery card from your graveyard to your hand. This cannot possibly target the card you just returned, since it's no longer in your graveyard to return.