The distinction between unread and unplayed (which leads to a lot of confusion) is being removed wherever possible in BP 4, and episodes will either be Played or Unplayed.

However, historically, BP always used to use a combination of Post Title, Post link and Published date to determine whether an episode was new. This meant that if the publisher corrected a typo in the title, or changed the location for the download, for instance, BP would think it was a new episode.

In the Advanced settings for the feed (in both 3.x and 4.x), there is a drop down choice to pick Title, Link, Date / GUID / Automatic for Duplicate Episode Detection. Choosing automatic will use GUID (a (supposedly) unique identifier) to determine whether an episode is new or not if the feed supports it. If it doesn't, BP will fall back to using Title, Link, Date.

Note that if you switch between TLD and GUID, BP has no way to know which episodes you have played, so you will have to manually mark them as played as a one-time exercise.