DuplicateUserNameErrorMessage doesn't work in CreateUserWizard

I have the following code for a DuplicateUserNameErrorMessage in CreateUserWizard, but it doesn't display the message. Suggestions? I'd prefer not to use code-behind unless absolutely necessary. Thanks.

DuplicateUserNameErrorMessage="Login Name belongs to another user. Please enter a different Login Name."

I've tried using this in combination with a ValidationSummary. I'd be interested in knowing of anyone who's been successful in doing something like this.
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