How would I check if a shulker box contains a certain item while it's still in item form?

[Danex]

I'm writing a /dupe plugin, (hold an item, type /dupe, it doubles. simple.) and I have some items blacklisted from being duped. However I'd like to allow shulker boxes to be duped to allow for making kits, but I can't figure out how to keep blacklisted items from being duped while inside a shulker. I'm using skbee to try and get the nbt data of the held shulker but I can't get it to work. Help?

[devyy]

If it's plugin when the player uses /dupe on a shulker box, loop through its inventory and check each item against your blacklist. If any blacklisted items are found, cancel the duplication and let the player know