How to Protect Your Excel Workbook when Saving the File
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Click the Yes button if the alert dialog box that asks whether you want to replace the existing file appears.
Select the Read-Only Recommended check box in the General Options dialog box instead of assigning a password for editing the workbook in the Password to Modify text box when you never want the user to be able to make and save changes in the same workbook file. When Excel marks a file as read-only, the user must save any modifications in a different file using the Save As command.