Diagnosis
The tax element &V1& already exists in the document with the key &V2&.

Procedure
In Customizing for TTE,
Choose Maintain Tax Calculation Procedure
Choose the relevant tax procedure.
Check that the tax element &V1& is uniquely assigned to the taxcalculation procedure. Adjust entries, if necessary.
Choose Save.
To update Customizing settings, choose ImmediateCustomizing Refresh.

Procedure for System Administration
I