Is the supplier's tax number a unique identifier？

After testing, the system will verify the uniqueness of the tax number when creating a new supplier;

However, when updating suppliers, the system allows changing the tax number of supplier A to supplier B's, meaning that the tax numbers of two suppliers are the same and the system will not report any errors.
I'm not sure if this is a bug. If not, why is the logic for creating and updating different?

Please help