Let me try to explain it a little better. I know categories cannot have the same ID. What I’m trying to do on many sites you’ll see a URL like this:
https://www.yoursite.com/index.php?cat=10+sub=15
So in this case, it’s loading a template from index.php and then looking for the category number 10 and the sub-category number 15 that is in the main category. This was sub-category 15 can always be the same topic. Let’s say Biography and it stays the same for each of the main categories.