Help With Procedure

Okay so, i have created a 3D model for a block called presser. The presser has 3 slots, 0 for inserting, 1 for inserting and 2 for output. I have already create the procedure of the presser how i want it to work, if you put to block of coal in slot 0 and slot 1, and press a button that presses them together and you get a diamond. BUT when i press them together i get the diamond, but if i dont take the diamond out before pressing again with 2 coal blocks, its still one diamond because it just sets one diamond in that slot when i press the coals together. Someone please help with this! Im stuck!