One of my users can open up documents that another user is in and it does not
display Read Only. The documents have not been set up as shared documents.
The only time that he knows that the document has been edited by someone else
or that someone else is in it is when he goes to save the document and it
asks if you want to overwrite the changes made by the other person or save a
copy of the file.

Does anyone know how to resolve this issue please.