As the title says, holding a bundle and throwing items in inventory GUI in creative mode have a chance to duplicate them.

If the items aren't properly displayed (like the end of the video), just open some other GUI like the crafting table and they will show up.

This duplication "feature" does NOT work if:

1. You are not in creative mode.
2. You don't throw the items through your inventory GUI (e.g., only press "Q").
3. You aren't holding the bundle when you pick up the items.

If non above is true but you just can't duplicate it, try put those item in your inventory rather than your hotbar and try again!

PS: Sorry for the low quality of the video, the original video is too big to be uploaded!