Planar Duplication

Orespawn-esque duplicator machine which duplicates blocks.
Duplicate responsibly.

Place a block on top of the duplicator and over time the 24 air blocks surrounding the duplicator in the 5x5 area will fill up with copies of the original block. (Only air blocks will be replaced.)

Iron blocks and ores cannot be duplicated, neither can duplicators themselves. And in versions starting from 1.14, emerald ores/blocks are also prohibited.