How do Strionic Resonator and Charmbreaker Devils interact?
You will get two cards.

The following describes what happens, in order:


Charmbreaker Devils's ability triggers.
Charmbreaker Devils's ability is placed on the stack. [Stack: CD]
You activate Strionic Resonator's ability targeting Charmbreaker Devils's ability.  [Stack: SR, CD]
Strionic Resonator's ability resolves.

Make a copy of Charmbreaker Devils's ability. [Stack: CD#2, CD#1]

Charmbreaker Devils's ability #2 resolves.

Return an instant or sorcery card at random from your graveyard to your hand. [Stack: CD#1]

Charmbreaker Devils's ability #1 resolves.

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).