Having more than one of the same item (i.e. using Diplopia and/or, in the case of trinkets, Smelter or Mom's Purse) will increase Isaac's damage for each copy.
Synergies
Mom's Box: Grants +0.4 damage for each evil item and trinket.
Golden Trinket / Mom's Box: Having either grants +1 damage for each evil item and trinket. Having both grants +1.5 damage for each.
Most items that are affected by Black Feather contain the phrase "Evil up" or "Sin up" in their description. The only exception is Goat Head, The Mark and The Pact.
Diplopia and/or, in the case of trinkets, 
Smelter or 
Mom's Purse) will increase Isaac's damage for each copy.
Mom's Box: Grants +0.4 damage for each evil item and trinket.
Golden Trinket / 
