You can duplicate copper blocks. This is how you do:
Create a world in 20w45a
Get 4 Copper blocks
Craft them into ingots to get 36 copper ingots
Upgrade to the newest snapshot
Craft all of them into blocks.
And you have 9 copper blocks despite hading 4 at the beginning.
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Yep just like @axoltl_stuff said, this is working as intended as the crafting recipe was changed in snapshot 20w46a.
Not a bug, snapshots are development versions and features therein are subject to change. You're not really supposed to play in snapshots unless you're testing the version in question. This issue doesn't exist when upgrading a 1.16.5 world to the latest snapshot.
This is because crafting a copper block went from 9 copper ingots to 4 copper ingots in a recent snapshot - this 'bug' is intended.