On the Java Edition you can Middle Click items (Select Block control) in your inventory, which will duplicate it into another stack of items which are exactly the same.
On the Bedrock Engine it does nothing at all, I see no reason to do this.
Some people say that it will make it unfair on mobile devices, but you can already select block on PC and not on mobile so this is not a valid argument.
(Note: this has obviously been the same before 1.2.13.54 but that is the oldest version it lets me select)