Boots Bug [GAMEPLAY]

I was playing as Garen, went full boots. Near the end of the game, when i was full build i tried adding a boot enchantment to my second pair of Ninja Tabi. However, when i tried to do that instead of giving the boots the enchantment the new enchantment overridden the first pair of boots.

The enchantment was different then the one on my first Ninja Tabi, and i didn't already have this enchantment on one of my other pairs of boots. It only happened when i tried it on duplicate pairs of boots.