Split from this thread.

Unfortunately I automatically delete all duplicates, so I was not able to  follow your advice (to reveal the message source) but I will do it soon.

However, other info is:

I use Windows Live Mail from Essentials 2012

Operating system is Windows 7

Two ISPs: One is iiNet.com.au  (IMAP) and the other is theory.com.au (POP3)

The duplicates are usually all in a row (6 or 7) and are received within one or two minutes, occasionally mixed in with other duplicates but usually all together.

I have twice tried the Repair function suggested in the Uninstall, Change or Repair program offered by Microsoft, but it had no effect. I also tried to reinstall without previously uninstalling, but I think this does not actually reinstall an already installed program.

I have also completely uninstalled Live Mail and reinstalled it, also to no avail.

This is apparently not an uncommon problem.

Some senders have let me know that their mail has been bounced, but in all cases I have still received multiples. 

I will get back to you when I have completed the Ctrl-F3 step you recommend.

Meantime, many thanks for your help.

Mikamble