When you say "sharing a workbook", do you mean "placing a workbook on a
file share where more than one person can edit it?"
It shouldn't be possible for two users to open the same document for
editing at the same time, but if this is what happened somehow, then
changes by the first person who saved might have been overwritten by
the second person who saved.
Again, I wouldn't think the file server would allow this: if it's a
bug, it's a bug in the file system sharing the workbook.
What you may need is a document management or revision control system
like developers use: only one person at a time can have the document
"checked out" for changes at at time. Once those changes are "checked
in", the other user can check out the workbook and make her changes.
Perforce is a popular commercial source code control system that offers
a plug-in for Microsoft Office for WIndows applications. WIth it, your
workbook versioning could be managed by Perforce from within Excel.
Performce costs about $800 per user, needs a server and takes some
administration, so it may be overkill.
Subversion is a very popular open-source version control system, but I
don't know much about using it for Office documents.
Then there's the old tried-and-true telephone system: call up the other
user and tell 'em that you're going to be making changes now, so leave
the file alone!
