Currently, the only attribute which has a Uniqueness Constraint option out-of-the-box is the sAMAccountName. If you configure the Uniqueness Scope in the "Built-in Policy - Default Logon Name", you could then configure a custom Policy which ensured that the Display Name contained the User's sAMAccountName, therefore ensuring that it would be unique.
It may also be possible to script a custom solution which would allow you to check for uniqueness without embedding the sAMAccountName, but I don't have a resource for that.
