It's not possible to create a user with the same full name in an OU.
To work around the issue, I'm trying to figure out how I can change the DN / CN for new users by default, and retroactively change old accounts.
Ideally, I'd use the email address.
I found this, but I'm not sure it does what I intend. Nor does it seem possible to use the email address field:
https://support.microsoft.com/en-ca/help/250455/how-to-change-display-names-of-active-directory-users