Oh, Bev. We may have fallen into the intellectual deep end here. And if you try to grab on to me, we'll both drown.

Your message could have been more efficient:

• "who say" already implies people, so you could have said "those who say" to be less redundant.
• "do so" is needlessly making a reference to exactly what you just said. Try "They say 'tuna fish' because".
• "someone else" is redundant because the only person that's not "else" is the person in question, and they wouldn't have heard it from themselves.
• "the kind of people to" is redundant because you already established that they're people. "the kind who" would be more efficient.
• "blindy follow others" doesn't need to say "others" because it's obvious that they'd be following someone other than themselves.
• "neither of those redundancies" is also a tad redundant, referring back to the established redundancies and then calling them redundancies again.

So a less redundant version of your message:

They say "tuna fish" because they heard someone say it, and are the kind who blindly follows rather than engage in critical inquiry and actively eliminate redundancy.

Intelligent people say neither redundancy.

Of course, I'm just poking fun. I don't expect anyone to eliminate all redundancy from their speaking; some of it has use, especially in verbal communication. For example, saying "datil pepper" even though datil also refers to the pepper is useful because someone may not recognize that a datil is a pepper upon hearing it (though you'd be hard pressed to find that scenario with tuna outside of ESL.)

I don't have a particularly strong distaste for redundancy but people blindly saying things without thinking nor any concern toward being readily understood is a bit of a peeve of mine. Particularly, I get offended when people expect one person to handle all of the effort for all sides of a conversation without supplying their sides' tools for doing so. Partly as an example, I have to wonder if you're getting downvoted by people who think that "n times as many as" "n times more than" are the same amount despite the obvious fact that "one time more than" is, by that standard, not at all more than the thing it's compared to. They just refuse to consider that what they're saying is plainly wrong and insist that people learn their wrong way of speaking. Grr.

Anyway, I assume I'm hopping onto the train to Karma Hell with you (as always; people who hate thinking really hate having their "thinking" questioned) so let's meet in the food car and get a sandwich together, yeah? You're already ahead on indignant downvotes so you can buy :P

Why isn't it Mumbai?

There's no such thing as a fish indeed.