In the most recent version of the FAQ (viewed 4/25/2014), in the section regarding multiple uses of the Belt of Incredible Dexterity, it states:
So you cannot reveal a single Belt of Incredible Dexterity twice to add 2 to your Dexterity check, but you could reveal two copies once each;
However, on p.15 of the rulebook it states:
Each player may play only one of each type of boon on a single check.
Was this (yet another) oversight? I don't have the card handy, so perhaps the card text overrides the basic rule?
Reveal this card to add 1 to your Dexterity check.
You may play another item on this check.
Some cards, like the Belt of Incredible Dexterity, have powers that allow you to break that rule. The FAQ entry was just to clarify that you couldn't create an infinite loop from two cards. The Belt of Incredible Dexterity says "You may play another item on this check." So, if you had two Belts of Incredible Dexterity could I play belt A, which grants me permission to play belt B, which grants me permission to play belt A, which grants me permission to play belt B, until my dexterity bonus reached 1,000,000,000,000,000,000,000 and I can't fail a check?
No. Because you have them not me. But even if I'd asked the question correctly about you having them and you playing them, you can play each individual card only once, though you are allowed to have 2 in your hand and play them both and a third, different item on the same check.