I made a flying machine but it seems to duplicate items but that is not the case.

It leaves a trail of observers slime pistons etc behind it but once the blocks are mined it drops nothing.

However if one would right-click it with an open hand the blocks will disappear.

If you right click the block with another block the block gets replaced with that block.

The screenshots will show you the build so you can recreate it and experience the bug for yourself.

In order to get the machine running use flint and steel on one of the two observers.