Will Kezan Mystic always destroy a secret you don't have?

Kezan Mystic has some specific interactions to avoid breaking some rules of Hearthstone:

1. If both you and your opponent have some of the same secrets in play, Kezan Mystic will always steal a secret you don't already have.
2. If you already have all of the secrets your opponent has, she will destroy a random one of theirs instead of stealing.
3. If you already have five secrets in play, she will destroy a secret instead of stealing.

In the situation in which you have 5 secrets and your opponent has some that you already have and some that you do not have, will Kezan Mystic always destroy one that you don't have (using a combination of rule 1 and rule 3)?

For example, let's say I have secrets [A, B, C, D, E] and my opponent has secrets [A, B, F, G]. When I play Kezan Mystic, am I guaranteed to destroy either F or G? Or is there a chance she will destroy A or B (which I already have)?

I just tested this in a duel. I had ice barrier, ice block, effigy, mirror entity, vaporize. Science partner had ice block, effigy, mirror entity, vaporize, counterspell. I played mystic, vaporize destroyed.

So it looks like it picks one at random to destroy.