You have two sessions of Word open at the same time and you attempt to save the current file with the same name as the file ...

You have two sessions of Word open at the same time and you attempt to save the current file with the same name as the file that is open in the other Word session. Find the other editing session and determine which version of the file you want to save. If you need to keep both, consider resolving the differences between the two files using the Compare and Combine commands on the Review tab in the Compare group.