TL;DR FeedMail will now ignore new items 7 days older than a previously seen item. This is expected to affect almost no "true" new posts. In theory checking to find new entries for a feed is a simple process. Download the feed. Check the ID of each entry to see if you have seen it before. However the real world is much messier. It is recommended for feed IDs to be URLs (to ensure global uniqueness) however this results in many feeds just using the URL that the article is available at. However these URLs sometimes change, and poorly designed feed generators update the ID of existing entries to the new URL. From a protocol point of view these are completely new entries, however to a user these are duplicates. In order to reduce the effect of this common issue on our our users FeedMail has some simple mitigations for posts that have recorded published dates. If the entry is older than a year always ignore it. If the entry is older than the 10th newest post in the feed ignore it.
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