When creating your Group, you can check On/Off the option to Allow Duplicates.Please Note: you can only do this at the time the Group is being created and not after it has been created. Go to Groups, New Group, chose the following :

GroupMail Format (Default), Next
Local (Default).. Next
On the Group Structure field press the Default Button to load the Default Group Structure or select the fields you wish to have in this Group, Next.
Email Field = Email, Next On the Group Structure field press the Default Button to load the Default Group Structure or select the fields you wish to have in this Group, Next.Email Field = Email, Next Here, enable the option to ‘Allow Duplicate Emails to be stored’ to allow duplicate recipients in your Group. Once the Group has been created you can then add duplicate recipients to it.