We realize it's quite common for page creators and added page users to have the same contacts. That's why if you or a fellow page user upload the same email address for a contact, our system will make sure the address receives only one notification email.
Additionally, you can view which contacts have been notified by all page users in your dashboard. This can be seen in the Notify Contacts section, under the People Who Will Know column. Contacts that have already been notified have a gray bar behind them (as pictured below).