Some 'accessories' - I believe the ROP was one - expressly allow you to play another card of the same type on the check (can't remember if the ROP does). However, there is a FAQ over at Paizo that specifically forbids that you reveal the same physical card, exactly to close such loopholes. Keep in mind, if you have TWO ROPs in your hand (and they state "You may play another Item on this check") - you should be able to play both.
The second part of ring of protection says that you may play another item on this check. So, a player should be able to use a second copy of ring of protection to prevent a second point of damage, but the card shouldn't be allowing a player to negate all damage. i tested this issue again against Iesha Foxglove. She dealt me 11 points of damage, and I was able to use ring of protection to negate all of the damage by revealing it 11 times.