How To Keep Windows From Opening New Copy Of Same Text File

I suppose I could use Word instead of notepad, but I don't want to wait forever for the files to open. Windows8 and 7 will let you open as many different copies of the same txt file as you want. With keeping my computer on most of the time, and doing various things, sometimes I forget that I have a file open. The most common instance is my list of software keys and activation codes. I had to reinstall something yesterday and had the file open, and it ended up behind a bunch of windows. Much later, I bought a new piece of software and had to add something to the list, a couple of somethings. I saved and closed--fine. Then, turning the computer off, it told me that my "Versions" file was still open--did I want to save, or what? At that point, I had to cancel the shutdown, open the saved copy, compare the saved copy with the one that was still open to see which information was different. Obviously, at that moment, I had not figured out which copy was which. If I had saved, it would have overwritten the changes, by reverting to the old version that I had open. I've had this happen several times, sometimes with new information in each file. I fail to understand why, when I try to open a txt file, Windows does not alert me that it is already open, and do I want to open another copy?