In creative, I dropped an anvil, and I picked up 2, dropped 2 individually, picked up 3, dropped 3 picked up 6. tried with diamond in survival, unsuccessful.
I did this while inventory screen was open, hovering over item, and pressing "q" the default drop button. This is all I know.
I did some testing and this came out.
In creative if an item is selected in the hotbar and "q" is pressed, the item will be thrown but not duplicated.
With an open inventory screen and one or more items in the hotbar and pressing "q" whilst hovering over the item and whilst another tab than the "Survival Inventory" selected, will delete the item.
The same but with the "Survival Inventory" open and the item in the "Survival Inventory" or the hotbar will duplicate the item thrown.
All of this doesn't seem to occur in Survival Mode.