Multiple file attachments have same content but different filenames.

Had a strange call from a member of staff today.

They showed me an email in their Sent Items (Office 2007) where they had attached 4 separate documents. I can confirm that, on the server, each document has a different filename with different contents.
The email was then sent to it's receiver.

Today I was called as they were told by the receiver of the mail that all the attachments, albeit with all the correct file names, all had the same content as one of the other files attached. Indeed each file was the same 60kb (same as the last document attached). So three files were wrong and one was correct in both file name and content.

So in front of me, a new email was created and all the documents were attached again. This time each had a different filesize and all were correct in their content.
The way the attachments were attached was correct (clicking on the Attach File button and browsing to file).

Any idea how such a thing can happen ? There are no documents on our server that have these same contents but different filenames. (e.g. Docs A,B,C and D all with different file names but with same content as D).
Docs A, B and C are all different and correctly so.

Can this only be a user error somewhere down the line or, somehow, can the software mess this up ?

Will check that out. Many thanks Ranger4