I found this bug with the amethyst cluster. If you put them on slime blocks (picture 1) and put sticky pistons behind the slime blocks and activate them via redstone. One of the Pistons will push with the amethyst (picture 2). and then deactivate the redstone and the piston will go back to his normal position the amethyst will drop the shard but the amethyst still remains (pic 3).
If you do this you can duplicate them easy.
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Hey there!
This looks to be a duplicate of MC-203561. This means this issue has already been reported before and is currently being tracked by the Mojira team.