One reason an Identity, email, customer ID, etc., can have multiple mPIDs if those identifiers are seen in different environments—Development vs. Production.
There are other reasons this could happen due to your ID Sync Strategy. Different strategies combined with the login, logout, and modify calls can result in this scenario, too.
User Activity View is a useful tool for looking at a user's events to determine what caused the profile to get created or updated.
Learn more about ID Sync with our docs.