Spoiler
Preparation: Use the most recent version of ADOM
1) Choose an inconsequential dungeon level > 1
- We will call this the target level
- I chose the carpenter level 2
- It works best if the level above has its 'up' and 'down' stairs in adjacent rooms
2) Drop an expendable item while on the target level, then climb the stairs
- We will call the level above the Source level
- As I said, this works best if the source level has its 'up' and 'down' stairs in adjacent rooms
3) Under your adom directory, go to the adom_dat\tmpdat sub folder.
4) Record the name of the most recently touched file
- This represents the level file for the target level
5) Next to the 'down' stairs of the source level, drop anything you do not want to duplicate, so that you are only carrying items you want to duplicate
- You should leave all your equipped items equipped, though
6) Stand on the 'down' stairs of the source level

Duplication Process
1) Drop all items
- ":d", "+" then "<enter>"
2) In the adom_dat\tmpdat subdirectory, delete the level file for the target level.
- You should have recorded its name in step 4) of the preparation
2) Descend the stairs
- Adom will detect an error, save your game then exit
3) Start adom again
- You will find yourself on the 'down' stairs of a copy of the source level. This copy includes the items you dropped, but ADOM will consider you to be on the target level.
4) Pick up all items
5) Climb to the source level
6) Goto 1)

Result:
Every time you drop your items in step 1), you will be doubling the pile of items (ie. adom duplicates the items of the source level every time it recreates the target level. You pick the copies up, then return to the source level, which still has the original items)