User administration - managing duplicate users

I have some users who have registered more than once using a different name but the same email address. 
This is generally accidental, it is quite easy to detect, but is there a way to merge them so one account is removed/deactivated and all of its posts get moved to the other account?  I don't want to lose the posts.

I have some other users who have registered multiple times using different names and email addresses.  These have mostly been banned but I would appreciate some more powerful tools to help spot this.  Email addresses don't seem to be directly searchable.  It would help if the list of users could be sorted by email address in various ways to help detect it - such as grouping by the same domain name and sorting by the email name (before the '@') regardless of domain name.
Also visibility of the IP address they used at time of registration and last login would be helpful.

There must be some differance in the email address as you can only have one account per email address.

Secondly there is no method of merging duplicate accounts. You can ban IPs of duplicate users, that's about the most powerful tool you have. You can also limit the number of accounts created under the same or similar IP addresses by going to ACP --> General --> Security --> IP address accounts creation limit, for each 24 hours --> change this to 2 --> Save

Thanks, I would have thought that must be the case too, but apparently that is not quite true:
(image removed now it has served its purpose to protect users privacy)

Any idea how to resolve it?

I'd say delete the 5 post one and cut the loss.

It isn't worth making a big deal about since the user isn't active on one of them. I think now though users can't use the same email address to sign up twice.

My other advice still stands though for the others who are creating duplicate accounts.

OK, I've just deactivated the 5-post one, because he did actually have recent posts on both.  
So if I delete the user all his posts vanish?  Or are they held in the thread by a phantom account?
It would be useful if there was a way to manage this properly.

Also it would be useful if the email sent when a new user joins had a bit more information - the username, their email address, and IP address ideally.  Perhaps it is configurable somewhere?  There are so many levels of options it's hard to be sure.

when deleting a user it gives you an option to delete their posts also if you dont delete the posts it simply says a guest posted them