Tweedledum and Tweedledee

UK: /ˌtwiːd^əlˈdʌm/, /ˌtwiːd^əlˈdiː/

US: (twēd′l dum′ ən twēd′l dē′)

WordReference Random House Unabridged Dictionary of American English © 2025

Twee•dle•dum and Twee•dle•dee  (twēd′l dum′ ən twēd′l dē′), 

1. two persons or things nominally different but practically the same;
   a nearly identical pair.

• 1715–25; humorous coinage, apparently first applied as nicknames to Giovanni Bononcini and Handel, with reference to their musical rivalry; see tweedle

Collins Concise English Dictionary © HarperCollins Publishers::

Tweedledum and Tweedledee / ˌtwiːd^əlˈdʌm ˌtwiːd^əlˈdiː/ noun

1. any two persons or things that differ only slightly from each other; two of a kind

Etymology: 19th Century: from the proverbial names of Handel and the musician Buononcini, who were supported by rival factions though it was thought by some that there was nothing to choose between them. The names were popularized by Lewis Carroll's use of them in Through the Looking Glass (1872)