Hello SAP S/4HANA Cloud Customer Community,
This query was raised with me earlier today - I will add the answer here in our community as well in case our community users have similar queries related to the changing of the info message which is raised when the duplication check for a Business Partner Tax Number Category is activated.
After activating the Duplication check for a Business Partner Tax Number Category the system will raise an info message for example if you were to enter a duplicate VAT number for a Spanish customer and you had set the duplicate check in the customising in SSCUI 102415 for ES0 as per your screenshot, then the system would raise this info message when a duplicate entry is entered "Another business partner already has this tax number (tax number type Spain: VAT Registration Number) Message no. BUPA_TAX400"
You could then even further restrict this by changing the message BUPA_TAX400 from an info message into an Error Message by using SSCUI 103007. BUPA_TAX would be the "Application Area" and 400 "MsgNo". This would prevent the creation of a second business partner with a duplicate VAT number.