[Feature]: Deduplicate entries across feeds

Feature Summary

Miniflux deduplicates entries within a single feed, but not across feeds. When the same article arrives from two different feeds, it appears twice in the unread list. I would like an opt-in option to hide cross-feed duplicates.

What problem does this feature solve?

I follow both an aggregator and the primary blogs it links to. Every article the aggregator picks up is delivered twice: once by the aggregator, once by the original blog. The same happens with wire copy published by several outlets, and with sites that re-promote an old article under the same URL.

The result is a noisy unread list and duplicate work. The only current remedy is manual: spotting the duplicates and marking them read by hand.

On my instance (Miniflux 2.2.15), I found 5 duplicate groups spanning different feeds: 11 entries, one URL appearing in 3 feeds. The aggregator was involved in every group. It is a small share of the total, but it is systematic and it grows with the number of overlapping feeds.

Current behaviour

Deduplication is scoped to a single feed. An entry is considered "already known" only if it matches an existing entry of the same feed. Two identical articles coming from two different feeds are therefore always stored as two entries.

Proposed Solution

An opt-in setting, disabled by default, that prevents an incoming entry from being stored when the same article already exists in another feed of the same user. The retained entry should keep its read status, and its content should still be refreshed if the source updates it.

Users who deliberately follow overlapping feeds must be able to keep the current behaviour, hence the opt-in.

Alternatives Considered

No response

Additional Context

No response

Checklist

• I have searched existing issues to ensure this feature hasn't been requested before.
• I understand that feature requests are not guaranteed to be implemented.
• I agree to follow the project's contribution guidelines.