In newer versions of Minecraft certain material types do not appear in inventory. Take sign as an example, if you try to duplicate a standing sign the type you get is SIGN_POST which does not appear in inventory, only the type SIGN does. Therefore Duplicator can't duplicate all the blocks we want.
Possible solutions:
Create a dictionary translating types to a type that can go in inventory (the power solution, not very elegant).
Detect that an item did not appear in inventory and rather give drop item (as drop item for these kinds of blocks tends to be the same as the one you click at).
Discoveries so far on this solution: When checking if inventory has changed (by comparing contents clone before and after item added), it seems the item is indeed added to the inventory, but does not appear in any way in-game. So finding a way to detect when inventory has actually changed is rather difficult.
In newer versions of Minecraft certain material types do not appear in inventory. Take sign as an example, if you try to duplicate a standing sign the type you get is SIGN_POST which does not appear in inventory, only the type SIGN does. Therefore Duplicator can't duplicate all the blocks we want.
Possible solutions: