Fake or Real Erasers.

How much you bought it? Or you gacha it?

The fake ones are from the silver/gold box event.

I still don't understand why we even have the fake erasers.

There are a ton of items which have 2 of the same item. They're used for quests too, just not the same quest as the other one. Like the Tachions for the Ludi quests. Each of the quests require Tachions, but the ones that are required are different in IDs/not the same item.