Custom Workbench Assistance

Hello. I made a custom workbench for my mod, and it works mostly-fine, the block only has one use and that is what I put in. You're supposed to put in one specific item in the input slot then in the output slot the other item will immediately appear, and its like crafting. Heres what actually happens: You put the item into the input slot, nothing shows up in the output slot, then if you click it you get the item then the item appears in there and you end up with double the amount of the item. I have tried countless solutions to solve this but I am stuck.