This occurs when one post has been published to multiple classes. Publishing a post to multiple classes is an advanced feature, which doesn't create multiple distinct copies of a post. Rather, it makes the same post available to different classes. And there is a single discussion underneath this single post. This is a design that allows a student's content to be shared for multiple audiences. For example, a student's post on the history of Frida Kahlo could be posted to an art class and a history class.
If a teacher does prefer that a comment from another class not appear via a particular class view, they can "hide" the comment. If the goal is to have the same content copied multiple times, with completely separate discussion threads the teacher should copy the same content and post to each class individually.