What is the cost of adding / removing the same user from AD B2C?

Question

I have a question about billing around adding / removing the same user in AD B2C. In particular "Azure Active Directory B2C (Azure AD B2C) pricing is based on monthly active users (MAU), which is the count of unique users with authentication activity within a calendar month." from https://learn.microsoft.com/en-us/azure/active-directory-b2c/billing?source=docs.

I am developing a functional test which will add and remove AD B2C users in a quick succession. There could be a 1000 in a batch which is added then removed within 10 mins. If the test adds/removes the same users then would there be no extra charge per month? If that's the case (no extra charge), what does the same/unique ("count of unique users with authentication activity within a calendar month") user mean? What makes a user unique, and how do I create the same/unique user?

Many thanks!

Thorsten