If you shift-click an item from your inventory to your hot-bar at the exact frame you pick up an item, the item will either be duplicated or it will simply be deleted. this works in survival mode (and in every game mode except spectator), so this duplication glitch should be patched as soon as possible before people start duplicating diamonds on servers.
This glitch may take many tries to pull off by the way (I've tried this about 25 times and I've only pulled it off around 3-5 times, one of them by accident while discovering this glitch.)
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We're tracking this issue in MC-29318, so this ticket is being resolved and linked as a duplicate.
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I regret to inform you that this is only a visual glitch.