Solutions
Prepare the bill. Do not post it.
Accountants need the invoice, the entity, and a warning when the same invoice number shows up twice. They do not need software that marks the bill paid.
The accounts payable draft compares invoice numbers inside the authorized workspace and leaves the match as a possible duplicate. A reviewer confirms the bookkeeping task.
Client separation in the live product depends on the organization and entity rules in the database policies. Those policies are written and statically checked. They are not yet running in a hosted database.