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What does this error mean 'Dynamics NAV: Error message: This is a duplicate of an existing Document'?


While attempting to calculate tax, this error message is given:

"This is a duplicate of an existing Document' with unique index 'IX_Document_Unique and cannot be saved."


Microsoft Dynamics NAV


  • This error message is caused when an invoice number in Dynamics NAV already exists as a DocCode in the Admin Console. 
    • Typically, the user is trying to release the document when this error occurs.  
  • To resolve this issue, the user will need to change the invoice number in Dynamics NAV to a number that does not exist in the Admin Console.


Note: This issue tends to occur when a live database has been restored to a test database.  The user is then generating sample invoices in the test database and the AvaTax Configuration settings are still pointing to the Production company in the Admin Console.

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