Step 1
Go to the player save file location
C:\Users\10K\AppData\LocalLow\IronGate\Valheim\characters
Find the character you are wanting to duplicate all items with.
Example: ottar geirleifs.fch

Step 2
Make a copy of the character file (I normally throw it on the desktop)

Step 3
start the game and log into a world with your character (preferably a solo world)
Throw everything you have on you in a chest or on the ground.
Then log out of the world and shut down the game. (its best to shut the game down so it reloads the character file)

Step 4
Go back to the character folder
C:\Users\10K\AppData\LocalLow\IronGate\Valheim\characters
replace the character file with the copy you have made.

step 5
start the game and log into the same world as before.(you will have all your stuff back)
Pick up everything you threw out (referencing step 3)

Step 6
enjoy the duplicates and game.