duplication glitch with sethbling hopperclock

using a sethbling hopper klock ( see attechment ) items will once every 3 hours on average create one extra. and stop the clock when it has happend

steps to reproduce
1 create hopper clock
2 let run for several hours.
3 pick up extra items

side note: hopper clock was connected to sugar cane farm.
fun fact: i put emeralds in cus i wanna get rich