Throwing items through portals can sometimes duplicate them

The bug

Items thrown into portals can be duplicated by travelling to the location you threw the items to twice. However, it only works when the items are thrown one by one and not when a stack is thrown.

How to reproduce

1. Create a portal and throw an item through from the overworld side
2. Travel through the portal and then back again, without picking the item up
3. Travel back through the portal and pick the items up
   →  The items are duplicated
4. (optional) Items that look duplicated but cannot be picked up can be picked up if you break the blocks under you and the items - you will pick them up while falling