Minecraft 1.8.3, Minecraft 1.8.8, Minecraft 15w45a, Minecraft 1.9, Minecraft 1.11.2, Minecraft 1.12, Minecraft 1.12.1 Pre-Release 1, Minecraft 1.12.1
Server duplicates items in hoppers. After a while, all hopper clocks are getting broken, because there is more than one item in it. I managed to duplicate diamond blocks.
Server Console says:
Edit: I now caught it happening. A hopper speed up so fast, that even a comparator doesn't work anymore. I'm sure this leads to item duplication.