When trying to update tax rules in Magento that have been imported by extensions such as firegento magesetup you might run into problems related to duplicate tax rule codes, e.g. “20% VAT” for both customers and retailers.
By default, Magento happily accepts duplicate tax rule codes when importing them trough setup scripts. But when trying to update these tax rules through adminhtml it will fail due to model checks with a “code already exists” error, as shown below:
The easiest solution here is to manually set the tax rule codes in your table tax_calculation_rule. Simply edit the code column there and you are all set. Happy tax’ing!