Usually the same bonuses does not stack with exception of dodge. However, in Master of the Wild, pg 36/ 37, it states "one ring of animal friendship adds 12 HD to the character's limit, a second raises that to 24, and a hand of glory allows the use of a third ring for a total of 36 additional HD of animal companions." This is different from the norm. If I were to wear two rings of protection +5, I would still only gain the benefit of +5 deflection. Why does ring of friendship stack? Are there any other such exceptions?