Steps to Reproduce:
1.have 2 unique{un-stackable) items in hot bar
2. using number, keys swap the two items
3. drop the new selected item
4. the player will drop a duplicated original item

Example
Coal in slot 1 diamond in slot 8
using number keys swap the coal into slot 8
now drop the coal from slot 8
coal has turned into a diamond
(also works for stacks of items)