excel no copy, users must use origonal

I have looked at some threads on preventing copies of excel files and all the help concentrates on high caliber protection. What I am looking for is, in a network, all users can edit the spreadsheet and save it, but only one copy ever exists.
My problem arises when a user creates a copy, usually inadervtantly when in READ ONLY mode and saves changes and then continues to use the copy which is not updated and therefore invalid.
This is not a security issue, it is simply to insure that all users are acessing up-to-date info.
In essence if the sheet is updated then saved, that is OK but if the user is in read only mode and attempts a save, excel will invite a new copy which I want to prevent. I only want one copy on the server and no copies in various MY DOCUMENTS.
My apologies for being long winded but most threads I read are an exersise in missunderstanding and I don,t need to keep out hackers. I just need one shared copy.

read only means just that. what is happening is the user is downloading a copy to their local drive and editing that.
this is not ever going to work.
use ;tools/share workbook
in advanced make sure the 'ask me which changes win is selected.'
then say a1 was edited by another user .it will tell them!
they must then select accept other users changes or it will prompt them to save a copy!

makes sense, thanks