Hmmmm. I'm not sure what you mean by "manually" run it. It has been a while since I looked at this code but I think I must have coded something that goes through the current playlist and adds an additional song for each artist that it finds. To fix this issue would require me to edit the code to first check if there are any other songs by that artist and then either add a song or remove the existing one. It should be doable but I'm thinking that this is not going to be a big priority for me at this point.
Here are some other options for you to consider:
1 - write your own MM script that does it the way you want. You can do this using VBscript I think, although I'm not sure how you would call it from VC. Possibly with a keyboard shortcut.
2 - First make sure your playlist contains only artists that have two or more tracks. You could do this with an sql query I think, but it would need to be part of creating a new nowplaying list, not by modifying whatever was already playing.
3 - write some VC magic macro that keeps track of what is playing and skips anomolous tracks when it encounters them. I'm not sure if this is possible because it would need to be able to look ahead to the next track to see who the artist was.
Something like: "if this track's artist is the same as the last track's, or artist is the same as the next track's then we are good, otherwise skip this track.