2 problems, help me please ;(

Hello, I have 2 doubts,first, how can I change the probability of drop that has a custom mob when dying, for example, I want my mob drop a helmet when he die, but I want to decrease the drop chances of the helmet, and the worst part, the mob drop 2 helmet, I dont know why, but he drop 2 helmets, I just want they drop 1 helmet, and change the odds what I can do to solve that?, and my second problem is , I managed to make that when you murder a certain mob, this acts like a matrioshka, and release another personalizated mob when it dies, this works properly, but he releases 2 of them,and I just want to release 1 of they, Thank guys, and sorry for my english, Im speak spanish and its hard for me