The Collaborative International Dictionary
Tweedledum and Tweedledee \Twee"dle*dum` and Twee"dle*dee`\ Two things practically alike; -- a phrase coined by John Byrom (1692-1793) in his satire ``On the Feuds between Handel and Bononcini.''
Wikipedia
Tweedledum and Tweedledee are fictional characters in an English nursery rhyme and in Lewis Carroll's Through the Looking-Glass, and What Alice Found There. Their names may have originally come from an epigram written by poet John Byrom. The nursery rhyme has a Roud Folk Song Index number of 19800. The names have since become synonymous in western popular culture slang for any two people who look and act in identical ways, generally in a derogatory context.
Tweedledum and Tweedledee are two fictional characters, a supervillain duo appearing in comic books published by DC Comics who were primarily known as enemies of Batman. The characters first appeared in Detective Comics #74 (April 1943), and were created by Bob Kane, Jerry Robinson and Don Cameron.
Usage examples of "tweedledum and tweedledee".
I forget their names, but thought of them as Tweedledum and Tweedledee, one fair, one dark, but identical in gaiety, indiscretion, and breezy but deferential attention to me - Tweedledee knew of me by name and fame, and was athirst for reminiscences, but since Tweedledum's interest was merely polite, and I'm an old hand at not being pumped, it was child's play to steer the conversation elsewhere.
Well, that square belongs to Tweedledum and Tweedledee -- the Fifth is mostly water -- the Sixth belongs to Humpty Dumpty -- but you make no remark?
They've gone home all right -- in the same way that Tweedledum and Tweedledee have gone home.