I can't have two enchantments on my armor that have the word protection in it.

I'm not sure if this is a bug but i find it annoying and think it should be fixed in a future update. While I was playing Minecraft I decided to enchant my diamond helmet with Fire Protection II, but my helmet already had Protection I on it. When I enchanted my armor in the anvil, what i expected to happen was that I would have Protection I and Fire Protection II on my diamond helmet. What actually happened was that it allowed me to enchant it but I still only had Protection I and the Fire Protection II did not go on. I went into creative mode to test more enchantments with the word "protection" in it and they did not work with a protection enchantment already on it. Again i'm sorry if this is not a bug and was meant to be in the game but it's annoying for me and think it should be fixed.

Thank You!