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Sorry if this doesn't make sense, but often when I do a search,
I see what appear to be obviously the same torrent (virtually the same name,
and same exact size, only from different search engines).
If these are truly the same torrents, wouldn't it be helpful to be able
to merge them when downloading? (often they all have different seeds/peers)
in order to sometimes complete otherwise incomplete torrents...

But that's how it works by default. It even says "trackers have been merged".

When you don't see that message, it's because they made a new torrent on purpose. A lot of the sites and communities disallow cross-seeding for example and this is why they have to make new, separate torrents. It's complicated, to say the least.