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1. Open Inventory Trade between Companions panels.
2. Left panel: Companion A; Right Panel: Player
3. Right Click desired item in Player inventory and move all stacks to Companion A inventory
4. Left panel: Companion B; Right Panel: Companion A
5. Right Click desired item in Companion A inventory and move all stacks to Companion B inventory
6. The items won't go to Companion B as expected.
7. Left panel: Companion A; Right Panel: Player
8. Notice that now both Companion A and Player inventories have duplicate stacks of the desired item
9. If you move the entire stack from Player to Companion A, you'll now have twice as much as you started with

Repeat the process until you have as many of the desired item as you want.

Step 5 can be repeated multiple times before proceeding to step 6 so you can multiply faster than just 2x if desired.

You can also juggle multiple items on steps 3 and 5 if you want to do things in bulk.

I personally prefer to repeat step 5 three times for each item so that I quadruple the item on each loop.

You can also duplicate high leveled items to be fodder for further upgrading at the anvil, tho eventually you reach an integer overrun which fails to level the item. But you can easily get level 1000 weapons with this method.

WARNING: There's an issue with this current public release that seems be causing the companion attacks to stall the game for a considerable amount of time (at least 30 seconds if not longer) if you've boosted your stats too high. I was able to reproduce this consistently by using 100 of each Essence on the Player, giving me 1000 in each stat. If I ditch the companions this stalling goes away, but I still run into stalls/crashes if I do too much growing. This particular aspect was more stable in previous releases. I suspect there's a nested function issue with the code involving stats/growth.

WARNING 2: The Dev has patch notes saying he intended to remove this dupe glitch. He has successfully removed the alternate dupe glitch with the anvil, but companion item duping still currently works. So have fun with it while it lasts. I'd personally prefer if he kept the glitch in for the time being as it allows the player to self regulate their own fun but also allows for easy stress testing of the growth mechanics, but the Dev is going to do what he'll do.