Unshared workbook allowing write access to more than 1 user
 

Does anyone know what causes the following problem ? I have an unshared

workbook that sometimes allows more than one user with write access to
open the workbook. Any changes made by one of the users will be
discarded with an error message "workbook has been update by other
users", although the file is not shared and has not been opened as
read-only.