Repeated conflicting changes reported in shared spreadsheet

I work for long periods on a large shared spreadsheet with one other user. When I periodically save I get the Resolve Conflicts dialog box showing some edits I have made in duplicate with identical author and values.

Sometimes the values are different. When the values are different it is not clear which entry is “Mine” and which entry is “Other” - the Author is identical - me!

I have not been able to discover the cause or the fix for this behavior. The same issue happened with open office.

Any ideas how to avoid the problem would be appreciated.