Flexi did not notify you of a duplicate received invoice? This is likely not an error, but rather the omission of one of the identifiers used for duplicate checking.
Flexi checks for duplicates based on two factors. The first is the incoming invoice number, and the second is the partner check in the address book.
In the address book, you have the option to track documents linked to a specific partner through that partner's record:
The duplicate check then takes place within these linked records.
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Example with a partner in the address book:
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Example with a partner entered without a record in the address book:
The invoice was saved without a duplicate check.