Received two combined emails as one

I use Windows 7 and Windows Live Mail.  However, recently, I have been receiving some old emails, which had previously been deleted by me and was also taken automatically off the server.

What is strange is that where the sender's names have been shown, it shows two different names of senders combined together.  I have never heard of this problem before.

Now my partner, who shares this desktop, but has a separate email address, is also receiving my old email with two email senders' names combined together on one email too.

Not sure if problem is with ISP and their server or Windows Live Mail. 

Any advice on how to stop this happening?

Thank you.