I'll admit the double foils happened only once to me (Regiment of Haradrim and Sword of Gondor in the same TTT Pack). Either it was a glitch or I was incredibly lucky! And I once also pulled 2 Arwens in one pack (though that was a Mount Doom Booster, not RotEL)!
To be honest, though, I'm more interested about the rarity of foil Promos. I'd have guessed Decipher put them on par with Uncommons (purely based on Boromir, DoMT being reprinted as one in Black Rider), but I'd like to know if anyone knows for sure.
I actually have uncut foil sheets from RoteL, and I can tell you that the premium foils are on the same sheet as the common foils. So, the probability of getting a foil premium is the same as getting any common foil (but they seem rarer, because for each premium foil on average you should go through all the common foils in the set as well) If the same is true for the 365 card sets, I don't know for sure, but I would assume as much, which would make premiums from these sets rarer since you have more commons to go through. So far as a probability for pulling a common foil vs an uncommon vs a rare, I have opened many booster boxes in my day, and I can say the average is about 3:2:1 for C/U/R. This to me makes sense in that there are on average 6 foils per booster box, each foil sheet I have has exactly 2 of each card on it, and all they would have to do is print 3 sheets/2 sheets/1 sheet in order to have these odds. Like I said, I am not positive on the odds, but it sure seems to me that it is this way.
I also recently purchased 36 booster packs from FotR, and for the rare foil I pulled a
Servant of the Secret Fire (which is awesome!) as well as a
gandalf's pipe foiled,
breeland forest foiled,
gondor's vengeance foiled, and
there and back again foiled. Here I pulled 5 foils, and it was 2:2:1 C/U/R (had I pulled one more foil, it would likely have been common)
So far as AI foils go, I have pulled many, and they are the rare foil that averages one per booster box.