A family of Microsoft word processing software products for creating web, email, and print documents.
Clambering in to the dusty recesses of my memory...
This can happen if someone emails you multiple documents with the same name, and you attempt to open them from within the email program.
If you always save the attachments to disk before opening them, you'll be a lot safer from Internet nasties, and the Finder will warn you if you are about to overwrite a same-named file.
Hope this helps