Maybe this from Debra Dalgleish's site will help:
http://www.contextures.com/xlfaqApp.html#AlreadyOpen

Blair wrote:

A friend has a problem. When he wants to open a file in Excel he finds the
file already open with a message which says the file cannot be opened as it
is already open.
I have tried to simulate this condition on my own PC but failed to do so.
Can anyone explain what is happening and how to stop it?
Blair

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Dave Peterson