i play on my sever with my friend and we both have the diamond upgrade i we just discorverd that we share it i tried multiple thing whith the shift right click menu but nothing changed

i think i did not do that but maybe i forgot but i will let my friend craft a new back pack to see if that works

It sounds like the backpack was duplicated via creative mode, or something else, which would cause both backpacks to have the same ID, which would cause them to share inventory.

it worked i could have sworn i crafted both at level 1 maybe that does the same thing but i least now we have our own backpack thank you

Creative mode duplicating is the only thing i can think of, as a new backpack, the first time its opened, picks a random UUID, which is a very big number, wikipedia

so if a backpack was duplicated after it was opened, you would end up with 2 backpacks with the same UUID, and thus the same inventory, 2 backpacks randomly picking the same number is improbably.