how to prevent a new notepad window from opening when the file is open

This is an annoying thing to me. After I open a text file using notepad, when I double-click the text file, another notepad instance opens the file. I can edit the file at both instances making a synch problem. (Sometimes I forget that file is already open which has not been saved, and re-open it and edit and save, destroying all the changes I made in the first notepad window.) Does anybody know how to fix this behavior of notepad? I want the existing notepad to be activated when I open the file again when it's already open. Thanks!