Finally, I understand. It was the first time I was using subcategories. I had moved topics from the parent category into the subcategory, and I assumed the parent category would only display its own topics, not those of the subcategories. That’s why there were duplicate topics between the subcategory and the parent category.
So, in the end, I don’t quite see the point of subcategories, since you could simply add a tag (representing a subcategory) to the relevant topics and then filter later to find them. I thought the parent category would only show its own topics and not those of the subcategories.