Items are duplicated using a hopper and chest or dispenser.

You will need 1 full stack of items to be duplicated, plus a few more of the item in a 2nd partial stack. In testing 9-10 seemed to be enough.

Step 1: Place a hopper with room for a chest/dispenser underneath it.

Step 2: Place a ring of blocks around the space above the hopper. Leave the center block empty.

Step 3: Place 1 full stack of an item to be duplicated in the first (leftmost) position of the hopper. Further, fill out the remaining empty hopper slots with stacks of any other kind of block. This step may not be necessary but was done during testing.

Step 4: Go back above the ring surrounding the hopper. Carefully aim at one corner of the blocks above the hopper. With the partial stack of items in your hotbar, hit Q until some of the items are stuck in the corner junction of the ring blocks and the hopper.

The items MUST be stuck and visible and not taken in by the hopper. This seems to be the nature of the glitch.

Step 5: Place a chest or dispenser under the hopper. Open the chest/dispenser and you should observe the duplication items filling in.

Step 6: Open the hopper and you should see the full stack of items in the first position do not depopulate.

Step 7: This effect will continue until the stuck items above the hopper despawn. It is trivially easy to create a feedback system that continues to refresh the stuck items using a water flow.