I found this while trying to verify another user's question how to clone user ? :
I was able to clone the user but the cloned user doesn't take on any attribute such as "group" that original user belongs to.
Also, I notice that there is also restriction on "Additional Email addresses". It doesn't allow same email address to be used by two different user. I imagine this to be useful in a corporate env when each support member uses their own email and have "additional email addresses" configured with a team notification mail queue so that there is a main notification queue that people can look at - I am looking at it from a support process angle.