We are currently rolling out this fix for all accounts in our 10.2.3 launch - you can ask your Account Manager when your account is scheduled for the update.
What you will see when setting up a New User is below:
Simply make sure that the checkbox is not marked if you wish to use an email address that is already existing on your RC account.
Now, currently this is only available for making individual Extensions - it is not available for bulk uploads just yet (something that RC is able to do for customers when asked). However, our Product Team is working towards a solution for bulk uploads as well! More to come on that!