Allow identical documents for different users (per-user private duplicate support)

Description



Currently, Paperless-ngx prevents importing identical documents based on a global checksum.
This works well for single-user systems, but creates limitations for multi-user setups.

Use case

> Multiple users in one Paperless-ngx instance
>
> Documents are private per user
>
> Users must not share documents
>
> The same file content may legitimately belong to different users (e.g. personal IDs, contracts, payslips, bank statements)


Example:

> User A uploads a document → imported successfully
>
> User B uploads the same exact file → rejected as duplicate
>
> User B cannot access User A’s document due to privacy rules

Also user A and B want to store the same document without sharing to anyone


Problem

Duplicate detection is global, not scoped per user.
This prevents true multi-user usage in a single instance.



Other

_No response_