Todd,
By doing what you want, you have to have unique email addresses for every user, which makes sense if you think about it.
Out of the box, DNN is set up to allow duplicate email addresses across accounts.
There is a setting in the web.config to enable unique addresses as a requirement - look for "requiresUniqueEmail" setting in the "AspNetSqlMembershipProvider" area of the web.config.
By enabling it there, you'll see some additional options/settings displayed in the site settings area for the site.
HTH,
Eric