Invoice numbering and suffix

Im trying to generate invoices from estimates and its using the same invoice number for 2 different invoices with different suffixes
Ive never seen this before
Ive tried to manually enter an invoice number but it says its been used - it hasnt

Any thoughts?

Has the invoice been created previously and then deleted? QuickFile never truly deletes an invoice to keep an audit trail (just in case HMRC asks).

By default we won’t add suffixes to invoices, if you add a suffix then we try to iterate the number from there. I would check your list of deleted invoices as there may be an invoice there that it’s colliding with. You can always restore this invoice and change the detail or simply change the number and re-delete.