Just follow the instructions on the video or read the short version below.
In a nutshell:
1. Go to the chest near the very beginning.
2. Open the chest, but do NOT pick up the item!
3. Get yourself killed.
4. Go back to the chest.
5. Pick up the item only ONCE, or it’s gonna disappear!
6. Get yourself killed again.
7. Repeat process!
Btw.: This is from the PS4. Can anyone confirm this glitch is working on other platforms?
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