Implement cross-feed deduplication with category merging for duplicate RSS items

Requested feature

Add global deduplication across all configured Feedzy RSS feeds (not just per-feed) to detect identical articles by content/URL regardless of originating feed. When a duplicate is detected in a new feed, instead of skipping, apply the new feed's categories to the existing post, enabling multi-category classification. Confirm current deduplication scope in docs/settings.

Customer context

I've configured several RSS feeds with Feedzy. In some feeds, the same article may be published. This means that the same article can appear in different feeds. ... if the same article is published in both 'feed 1' and 'feed 2', is it detected as a duplicate? If a duplicate is detected, is it possible to apply its category to the already existing post, the post would then be classified in 2 categories.

Triage assessment

• Impact score: 65/100
• Confidence score: 90/100
• Complexity score: 55/100

Source

• HelpScout conversation: https://secure.helpscout.net/conversation/3283712016/487464
• Conversation ID: 3283712016

The payload names no implementation files or tests; start by tracing the existing per-feed deduplication path and the post/category import entry point. Confirm the current deduplication scope in the documentation or settings, then verify that cross-feed duplicates are identified by content or URL and that categories from later feeds are merged into the existing post.