Some downloaded feeds contain multiple entries for the same message.
See `http://feeds.feedburner.com/abseilio` for an example that has:
```
<pubDate>2020-09-11T00:00:00-04:00</pubDate>
<link>https://abseil.io/tips/5</link>
--
<pubDate>2020-06-01T00:00:00-04:00</pubDate>
<link>https://abseil.io/tips/5</link>
```
When updating the database for the first time, both messages end up
stored. The following feed updates result in one entry matching the
database, and the second entry having a different creation date. This
makes the second entry always marked as unread.
