Not sure if this is the reason, but you may have enabled shared mode.
Check Tools | Share workbook...

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Regards,

Tushar Mehta
www.tushar-mehta.com
Excel, PowerPoint, and VBA add-ins, tutorials
Custom MS Office productivity solutions

In article , "=?
Utf-8?B?U2hhcmkgS29sYm8=?=" <Shari
says...
Each time I save an Excel File, the following message appears and I want to
disable it.

The file may have been changed by another user since you last saved it. In
that case, what do you want to do?

Save a copy
or
Overwrite Changes