Hey,

there's a dupe glitch with Conveyor Belts and default Hoppers.

Exact explanation can be found in this video: https://www.youtube.com/watch?v=TXcxwR36HA4

Possible fixes would be disabling the ability to put Conveyors on Hoppers, make it impossible for Hoppers to pull items through the Conveyors, give a config option to disable item movement by Conveyors.

Sincerely,
Rakinare