Workbook Merge Fails

[Brian Dugas]

This is the error message:

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That workbook cannot be merged because it contains changes
that have not been merged followed by later changes that
have already been merged into the current workbook.
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I turned on workbook sharing. I made a copy of the Excel file, and
distributed to a few different people. I call the original copy the master.

I get this error when trying to merge one of the copies back into the
master.
What causes this error?

Thanks.
Brian