You saved a received invoice that you already have in the system, and Flexi didn't warn you about the duplicate. In most cases, this isn't an error — one of the fields used for the check simply wasn't filled in.
How the duplicate check works
Flexi compares two pieces of information at the same time:
Received invoice number on the invoice
Partner selected from the address book
The check is performed on the links that the partner has in the address book — for each partner, you can see the documents linked to them, and it's within these that Flexi looks for a match.
Partner selected from the address book
Flexi will find the duplicate and warn you about it.
Partner entered manually, without a link to the address book
The invoice is saved without any warning. Without a link to a partner, Flexi has no basis for finding a match.
💡 If you want to be able to rely on the duplicate check, always select the supplier from the address book and fill in the Received invoice number field. Manually overwriting the partner's details directly on the document will disable the check.