[Bug] Dynamic OPML Duplicates Feeds

### Describe the bug

When I use Dynamic OPML to keep a category synced up with a published list, it sometimes repeatedly duplicates feeds. Generally everything is correct after first import, but roughly 24 hours later some of the feeds are added again (and again 24 hours after that)

Only some feeds are affected by this, so far it appears that Wordpress feeds that use `http://example.wordpress.com/?feed=rss` as the feed URL get duplicated. Similar Wordpress blogs using `http://example.wordpress.com/feed` appear to be unaffected.

### To Reproduce

1. Add an OPML file as a dynamic source that includes wordpress blogs using the indicated structure (for example: https://hobbies.deimos-legion.net/wp-content/uploads/static/hobby-blogs.opml)
2. Wait 24 Hours
3. Notice that several feeds are now duplicated.

### Expected behavior

When a dynamic OPML source is added, each feed appears only once

### FreshRSS version

1.28.1

### System information

Hosted on a pikapod. Container seems to be up to date.

### Additional context

_No response_