Mark Twain? Biblical Proverb? Apocryphal? Anonymous?

Question for Quote Investigator: Conflict on social media is now endemic, but it is not really new. Acrimonious exchanges between participants during the primeval days of online forums were known as “flame wars”.
Famed humorist Mark Twain has received credit for a germane cautionary remark:
Never argue with a fool; onlookers may not be able to tell the difference.
Unfortunately, no one has presented a good citation for Twain. Would you please examine this saying?
Reply from Quote Investigator: QI has been unable to find substantive evidence crediting this remark to Mark Twain. It does not appear on the Twain Quotes website edited by Barbara Schmidt,1
nor does it appear in the large compilation “Mark Twain at Your Fingertips” edited by Caroline Thomas Harnsberger.2
The Bible contains a thematically related passage in Proverbs 26:4 and 26:5:3
Do not answer a fool according to his folly,
Or you will also be like him.
Answer a fool as his folly deserves,
That he not be wise in his own eyes.
Statements that were closer to the modern template emerged in the 1800s. Here is a sampling with dates which shows the variation in phrasing and the evolution over time. All of the earliest citations were anonymous.
1878: Don’t argue with a fool, or the listener will say there is a pair of you.
1878: Don’t argue with a fool or listeners will think there are two of you.
1896: Arguing with a fool shows that there are two.
1930: When you argue with a fool, he’s doing the same thing.
1930: When you argue with a fool be sure he isn’t similarly occupied.
1937 Never argue with a fool. But if you must, the safest way is to carry on the debate with yourself.
1938: Never argue with a fool in public lest the public not know which is which.
1943: When you argue with a fool, be sure he isn’t similarly engaged.
1951: It isn’t smart to argue with a fool; listeners can’t tell which is which.
1954: Never argue with a fool. Bystanders can’t tell which is which.
1966: Don’t argue with a fool. Onlookers may not be able to tell the difference.
1999: Never argue with an idiot. You’ll never convince the idiot that you’re correct, and bystanders won’t be able to tell who’s who.
2012: Never argue with a fool; onlookers may not be able to tell the difference.
Below are selected citations in chronological order.
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