Filter on duplicates by linked URLs

In addition to the filter request, I would love to get an feature, that help to reduce the amount of items. It should merge different feeds by eliminating items that contain the same links to external third party pages.

For example: You subscribe two feeds of newspaper A and B. Within feed A is a article item, that text links to reference 1. Also at feed B is a (diffrent) article, that links to reference 1, too. The feature now marks feed B article as read, to avoid the dublicate.

This feature might become complex due to blacklisting URLs (think about embedded HTML widgets, ...), suppression priority, ...

The issue does not name files, tests, or entry points; begin by tracing feed-item filtering and how linked external URLs are represented. Clarify duplicate matching, URL blacklisting, embedded widgets, and suppression priority before defining what marking an item as read should mean.

Written by the indexing model from the issue text.