"allow_multiple_emails" system setting not working

I seem to be able to create multiple users with the same email address despite having ’allow_multiple_emails’ set to ’no’. Both in RC-1 and RC-2, and both using the Register snippet and using the Manager Add User interface. I could swear that in the past I’ve seen an error/constraint pop up that forbid me from doing this. Could this have been working back in beta 5 and not be working now? Or am I just hopelessly confused? (Wait - don’t answer that).