Check if Post by Email is a Duplicate

Received a feature request:

> It would be great if we could run a trigger to evaluate if the incoming post is a duplicate and stop it being added and being shared again..
> We can trap the duplicate by subject for example

Possible action would be to set the duplicate post as a draft, so it can be reviewed before publishing if it indeed was a separate post, or deleted if it was a duplicate.

3316399-t

No file, test, or specific entry point is named. Start by tracing the Post by Email ingestion flow and its existing trigger or moderation hooks; done should include a defined duplicate criterion and a documented behavior for marking a duplicate as draft, deleting it, or allowing review.

Written by the indexing model from the issue text.