Meaning of “But I repeat myself” in Mark Twain's quote?

There is the following sentence in the conversation between Florentyna Rosnovski, the heroine of Jeffrey Archer’s novel, The Prodigal Daughter, who was first elected as the Congressman of Illinois and her husband, Richard Kane, Chairman of a New York bank.

She captured the Ninth District of Illinois with a plurality of over 27,000 votes. Richard was the first to congratulate her.
"I’m proud of you, my darling." He smiled mischievously. "Mind you, I’m sure Mark Twain would have been as well."
"Why Mark Twain?" asked Florentina, puzzled.
"Because it was he who said: 'Suppose you were an idiot and suppose you were a member of Congress. But I repeat myself'." – The Prodigal Daughter, Page 302.

I can’t get the idea of the last phrase, “But I repeat myself,” following the preceding subjunctives – Suppose you are idiot or a member of Congress. Is Richard simply saying “Whoever you are, I won’t change.”?

What is the meaning of it as the punch line of a famous quote from Mark Twain?

Is he saying he won’t change his attitudes / belief / way of life whoever the counterpart is, i.e. he always stays as he is? Though it may sound uncouth, what is the essence of the humor of this line?

Solution 1:

The joke is that Twain considers idiot and member of Congress to be synonymous!

Solution 2:

Mark twain's comment relies on the sarcastic use of tautology:

needless repetition of an idea, statement, or word.

The author Dan Brown mistakenly makes a lot of tautological statements, parodied in this description:

The critics said his writing was clumsy, ungrammatical, repetitive and repetitive.

The last word is an unnecessary repetition of repetitive - a tautology.

What Mark Twain is saying is that to say suppose someone is an idiot and suppose someone is a congresman is a tautology: they are the same thing. (All Congressmen by his definition were idiots.) To say both is a tautology.

To be honest, the writing is poor in the section you quote, because Richard is basically saying that Mark Twain would have considered F. an idiot, which is not something Twain would have been proud of. So Richard either is insulting his wife, or he is really not making sense here, which might be why it's confusing.

It's just a way for the author to sneak in a brilliant quote without having to work for it. It's an example of sloppy writing.