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I keep my webs on a completely different machine and then publish to the server. If you're opening and editing files on the live server then they are going to overwrite the existing content.
In your case:
\\intranet\marketing\test.aspx
http://intranet/marketing/test.aspx
These are both the same address when accessing the files locally. It would be best for you to make a separate staging area where you could do development testing if you don't want this to happen.