Why not enter old invoices in the system? What does "duplication" mean?

Example: in September you gave a customer invoice no. 50 for 10,000 (on paper or from an old system) and paid its VAT. Today you enter it in the system for a complete record, and the system sends it to ZATCA as a new invoice issued today.

Result: the work happened once, but ZATCA now has two invoices for it and sees 20,000 of sales instead of 10,000. That is "duplication", and the second invoice can only be undone by a credit note.

The right way: don't enter the old invoice; enter only what the customer still owes as an opening balance (question 14).