If you have a hopper connected to another hopper which is connected to a dispenser (can have more hoppers than two) and throw a piece of coal on the back edge of the first hopper (use a wall) it will not pick up the piece of coal but will repeatedly intake coal.
Look for the coal pillar in the world download.
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I did search. It is not a duplicate, it only works in very specific circumstances that are not stated in the other. Also the other duplicates one every time this one duplicates repeatedly.
All duplication glitches with the hopper are summarized in MC-5998. If you have found a different method, please leave a comment there.
Duplicate of MC-5998 , please use the search function to see if your bug has already been submitted. Currently over 40% of tickets are being closed as duplicate.