I accidentally found a way to duplicate redstone blocks.
Steps to reproduce
1. Place a redstone block two blocks away from the ground
2. Then place a piston on the ground right below
3. After before the piston can extend place any block above it resulting in a duplication of the redstone block.
4. Then update the block that has been pushed up which will then make the block disappear and be replaced by redstone.
I expected the piston to not even extend at all.