STEPS TO REPRODUCE
1. Deploy Kaleo
2. Login as the admin and change the admin email address to an address of an account you can receive emails
3. Add role to user which can approve submitted content - Administrator role is one but add another one such as the regular role portal content review role (check single-approver-definition.xml for all the available roles for reviewing content)
4. Configure an email server
5. Create new site
6. Enable workflow for the site's web content
7. Create a web content and submit for publication
8. Notice in the body of your email you will see that your admin user will be sent email twice. If you are using gmail, you need to select "show original" - see screenshot - to see how the email is sent multiple times.
The notification is will contain the admin user multiple times as the recipient.
Each unique recipient should only be listed once.