SAP Message 8B264 - Nota Fiscal number &1 is not unique for partner &2

Diagnosis
The nota fiscal number &1 is not unique for partner &2. A nota fiscal from the same date already exists.

System Response
Material document - nota fiscal interface error.
The material document is nota fiscal relevant and requires aunique nota fiscal number.

Procedure
Enter a different nota fiscal number in the Delivery Notefield in the material document.