There are most likely several ways to reproduce this but this is how I did it to confirm item ghosting duplication through connectivity issues.
1) Put item in your hands
2) lose connection (unplugging network cable)
3) drag&drop move item from your hands into inventory slot A
4) move same item, that is still in your hands, into inventory slot B (different from A)
5) regain connectoin (plugging network cable back in)
6) deselect item from your hands by clicking the X icon
7) move item that is now in slot A into slot B
Now the item should exist in both item slots A and B. Whereas one seems to be a fake item and its slot cannot be used anymore manually.