Duplicate post of Parent and Child categories

I’m new to magazine style themes that display widgets in the content area that are category driven. I installed the My Category Order plugin and changed my template $cat_args code so that the orderby parameter was “order” instead of “name”. This allowed me to use the plugin to change the order in which the category widgets appeared on the page. It’s working perfectly. The category name appears in a title bar above the post.

I hit a snag when made a post and assigned it to a child category. Now that post appears in its own widget, with its name in the title bar, and it also appears in the parent category widget. The child widget appears directly under the parent widget. So now I have two posts on the page that are exactly the same.

I’ve been looking for plugins or code that I can use to stop the duplicate posting. What I would like is to have the child post in the parent widget in the main content area, then have it listed as a child in the sidebar Categories widget.

Every solution I’ve been finding online simply excludes the child category all together. I want it to show, but in the parent widget.

I did install the plugin Just One Category and it had no effect at all, but that may be due to the changes I had previously made in my site code.

As popular as magazine themes are, I’m shocked not to easily find a fix for this. Any help or suggestions are most welcome.