Minecraft – How to duplicate blocks

Hold the Ctrl key (instead of cmd) then press the pick block button (mouse wheel click by default) with the player head highlighted.

This can be done via the /fill command within /execute.

/execute @a ~ ~ ~ fill ~10 ~10 ~10 ~-10 ~-10 ~-10 pumpkin 0 replace dropper 1

/execute @a ~ ~ ~ fill ~10 ~10 ~10 ~-10 ~-10 ~-10 dropper 1 replace pumpkin 0

You don't have to use a pumpkin, any unused block will do. Put these two commands separately on a fill clock and run a comparator out from either of the two command blocks with the commands in them. Whenever any player is within 10 blocks (including diagonals, unfortunately) of a dropper with the data value of 1 (was that facing down? I can't remember), the comparator will turn on. Whenever they move out of range, it will turn off. That's currently the closest you can get to what you asked.