Podcast app plays episodes I've already heard

Lately after one podcast on my list is played, the app will sometimes play something else I've already listened to - almost always the same one show. If I mark that one as played, it goes on to another older episode in that same show - which I've also listened to. And so on.

How do I get the app to consistently ignore episodes I've already listened to? Or is this a bug?

This has been happening occasionally for at least a year, but I've had this issue more lately.