Why is a "1" appended to the username of our newest user?

We have a new user who just signed up and his username is mhuberman

However his profile page is https://www.drcarney.com/mhuberman1

Why is that? If you login to the backend he has selected mhuberman as his username. So, we are confused about the 1 at the end.

To reproduce this you can login to our site and visit the users page at https://www.drcarney.com/community/users

Then click on mhuberman link under his photo. I would expect this link to take a person to https://www.drcarney.com/mhuberman but instead it goes to https://www.drcarney.com/mhuberman1

Both of those URLs show the page. But, it seems like the one with the 1 at the end should NOT exist.

Thanks, Sean