Duplicate posts in feed

[Barmar]

When I view the following feed:

http://www.bridgewinners.com/index.php?format=feed&type=rss

I frequently see articles that I've marked as read reappear. And if I
turn off "Hide Read Items", I see lots of duplicate articles in the
list. But I don't see duplicates if I look at the XML source.

I'm not using any form of syncing, just NNW 3.3 on a Mac.

If this is a problem with the feed, what should I tell them they need
to fix?

[Seth Elgart]

I've seen this happen occasionally. A site may update or correct an article which might make it get sent out in their feed again. Some sites may "clean" their feed so you don't get multiple entries but some sites may not, of if you've gotten your feed already and then they make a change you might get the same article again the next time you refresh. 

I'm not saying this is definitely the cause, just that it's a possibility. I've seen two "identical" articles in a row in a feed occasionally but where they've changed the headline from some awkward wording to a better one, for example, or maybe they've fixed a typo that got caught right after it was posted. 

Seth

[Barmar]

I've looked a little closer, and I think I see what's happening. The
URLs that the items link to are slightly different. When I step
through the articles in a set, the URLs will be:

http://bridgewinners.com/whatever
https://bridgewinners.com/whatever
http://www.bridgewinners.com/whatever

where "whatever" is the same for all of them. This seems to happen for
almost every article (although some only get 2 of the 3). What could
they be doing in their blogging software that causes the URLs to
change from day to day?