You just looted a 2.0 second . With haste, you open your inventory to equip it. What happens next? Nothing, that’s what. You are greeted with a message that states “You cannot have more than 1 Wrathful Malignant Heart equipped.”
Currently, the only way to fix this is to un-equip your piece of jewelry to replace it with your new Malignant Heart. The good news is, in Patch 1.1.2 this will be fixed, and you can directly replace your Wrathful Hearts.