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.)