The spice you don't have should go from 0 to -1 quantity and appear in the list of spices being used on the top screen.
Before the inventory screen appears, the hero should bend over to check the ground. Select the desired trap from your inventory and 'use' it. You should here the sound indicating that you've picked something up, while your hero is still bent over. The trap you just placed should be in the last slot of your inventory, as if you just picked it up. You will not be able to see the trap on the floor, however, leave the area, come back, and the trap will have been placed. Continue to do this as many times as you want to duplicate traps.