note: this all happened after i exited a nether portal

if you place a block down (might be just shulker boxes, it happened with those) in a singleplayer world, your computer dies after they get placed, and you rejoin, you spawn at the portal spot with the full (items previously inside are kept) blocks placed down where they were but also a clone of the block is in your inventory

also, since the file size limit is reached (i recorded a short video to demonstrate the duped items) i will put a link to a Medal clip, using the same video

https://medal.tv/games/minecraft/clips/r9w7EI9wgM9Wc/d1337iMD7OV1?invite=cr-MSxjcFMsMzM0NDAwNzEs