I'm unable to have two of the same backpacks. Then I put items in one backpack, they appear in the other backpack as if they are linked. Noticed this with two gold backpacks I had gotten in survival. Spawned in two others to confirm.

Check UUID of those backpacks in advanced tooltip (F3 + H). I am pretty sure it's going to be the same. And if so I will need to understand how you've got it and steps to recreate getting the backpack with the same UUID. As far as I know it's only possible to get them like that by duplicating in creative as you did to confirm. But if there's a way to do that in survival I can take a look.

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