hadn't thought of that and you're right, MC is happily auto importing files it created itself. I can't imagine why that is desirable behaviour, surely a bug?
In the auto-import settings you can configure MC to import or not import those kinds of things, so I'm not sure it's a bug (someone may actually want that behavior?). For example, you can tell auto-import not to import image or data files in video import directories to prevent that behavior. I have JRiver configured to never import images except in directories where I deliberately store images and none of my JRiver created cover art is imported into my library, etc.
The default behavior when you add an auto-import directory is to import everything, which does lead to this effectively being the default behavior, which is a little odd, and leads to confusion about this kind of thing (also to strange results when using the rename copy tool, etc.). But it's easy enough to configure away.