Duplicate invoice numbers

How can an invoice number be assigned twice?

Duplicate invoice numbers typically occur in the following scenarios:

Scenario 1: Manually assigned numbers

If you enter invoice numbers manually (not generated automatically):

• You create an invoice with the number "RE-2026-0001"
• Later you create another invoice with the same number "RE-2026-0001"

The system accepts this under certain settings.

Scenario 2: Numbering with gaps

If your number assignment is not continuous:

• You create invoices with the numbers 001, 003, 005 (gaps)
• Later you accidentally enter 003 or 005, and a duplicate number is created

Scenario 3: Several invoice series

If you use several number series:

• Series A: RE-A-001, RE-A-002, …
• Series B: RE-B-001, RE-B-002, …

This should prevent duplicates if the prefixes are different.

Can the system prevent duplicates?

Yes, under certain conditions:

For Peppol invoices (format: "peppol"), the invoice number acts as an idempotency key:

• The API prevents the creation of a second invoice with the same number
• If you try, you receive an error (HTTP 409) with the ID of the existing invoice

However, this safeguard does not exist for all formats (PDF, ZUGFeRD without Peppol).

What to do about a duplicate invoice number?

Solution 1: Delete one invoice

If the duplicate number was a mistake:

1. Decide which invoice to keep
2. Delete the other invoice (requires the DELETE_INVOICES permission and possibly invoiceDeleteMode)
3. If that is not possible: cancel one of the duplicate invoices

Solution 2: Cancel one invoice and create it again

If both invoices are legitimate but one is incorrect:

1. Create a cancellation invoice for the incorrect invoice
2. Create a new invoice with a different number

The cancellation is audit-proof and leaves complete documentation.

Solution 3: Check the number format

If duplicates keep occurring:

• Check your number assignment settings in faktoora
• Make sure that the number series is continuous
• If necessary, use different prefixes for different invoice types

Audits by authorities and duplicates

If you have to report duplicate invoice numbers to authorities (e.g. during invoice audits):

• Document the cancellation invoice that corrects the error
• Show the correction in your accounting
• Explain why a number was assigned twice

This is audit-proof if you have documented it with a cancellation.

Can I detect duplicates afterwards?

Yes. In the faktoora invoice list, you can:

1. Search for the invoice number
2. Several hits mean: a duplicate exists

If you want to check for duplicates regularly:

• Use the search and filter function in the invoice overview
• Export invoices and check for duplicates in a spreadsheet

Confirmation dialog "Invoice number already assigned"

If you try to create an invoice with a number that has already been assigned, faktoora displays a confirmation dialog titled "Invoice number already assigned". This dialog has no close icon; you must actively leave it via one of the two buttons offered to continue creating the invoice:

• Confirm: Applies the next free invoice number suggested by faktoora (with a custom number format, you can still adjust the suggested number in the dialog) and closes the dialog.
• Cancel: Closes the dialog without changing the invoice number. The invoice remains a draft so that you can correct the number yourself.

Prevention: choose the right number series

The best solution is to prevent duplicates:

1. Choose a clear numbering system (e.g. RE-YYYY-XXXX for year and number)
2. Let faktoora number automatically (not manually)
3. Use different series for different document types (invoices, credit notes, quotes)

This significantly reduces the risk of duplicates.