As a merchant, how do I dispute a false claim of multiple listings for the exact same item?

[Amethyst Jetaime]

You need to file a support ticket and explain all this to LL.  They are the only ones that can unlock your item and put it back on the MP if they agree with your explanation. 

You can do this for any marketplace issues that requires their assistance, even if you are not premium.

[Amethyst Jetaime]

You need to file a support ticket and explain all this to LL.  They are the only ones that can unlock your item and put it back on the MP if they agree with your explanation. 

You can do this for any marketplace issues that requires their assistance, even if you are not premium.

[Lindal Kidd]

It's not that you only have the one copy.  They get upset if you list an item, then delist it, and list it again.  That's what they mean by "multiple listings for the exact same item."

This policy was implemented long before gacha, and people selling unused gacha prizes.  You'll have to file a support case and explain that you've obtained another copy of this No Copy item and you're offering it as a new, separate item.