EDIT: thanks for the link. Please open that in a normal browser tab so that you can see that elements don't have [pubDate] or [dc:date] tag (which they should have since the feed claims to be RSS 2.0). Due to this Feedbro generates timestamps for the item (article) entries. But when you delete such loaded article, Feedbro has no way of knowing whether the article was loaded before (because it's now removed from the Feedbro database). This is why Feedbro downloads the article again.

Solution: don't delete articles that are provided by such feeds (that don't have datestamps).

What's the feed URL? If you are deleting articles and the feed does not date stamps for the articles in the XML, that can happen.