What do you mean by unique?

From https://support.nwayplay.com/hc/en-us/articles/4402815973399-I-see-that-there-are-multiple-pins-with-identical-designs-so-why-are-you-saying-that-each-one-is-unique

I see that there are multiple pins with identical designs, so why are you saying that each one is unique?

Each pin has its own unique serial number. Similar to other collectibles, collectors pay close attention to serial numbers. Thus, the same pin with different serial numbers can have different market values.

Keep this in mind when you’re looking to resell your pin on the Marketplace.