“It is the soothing thing about history that it does repeat itself.”

As quoted in Peter's Quotations: Ideas For Our Time (1977) edited by Laurence J. Peter, p. 248

" Some Historians http://books.google.com/books?id=E0luAAAAMAAJ&q=%22History+repeats+itself+historians+repeat+each+other%22&pg=PA20#v=onepage," Supers & Supermen: Studies in Politics, History and Letters (1920)

and lets fly with a club.
Statement in Analog Science Fiction/Fact magazine (1965)

Niven's Laws

Source: Reminiscences of a Stock Operator (1923), Chapter XVIII, p. 217

Augustus (1937)
Context: History does not repeat itself except with variations, and it is idle to look for exact parallels, but we can trace a resemblance between the conditions of his time and those of to-day. Once again the crust of civilization has worn thin, and beneath can be heard the muttering of primeval fires. Once again many accepted principles of government have been overthrown, and the world has become a laboratory where immature and feverish minds experiment with unknown forces. Once again problems cannot be comfortably limited, for science has brought the nations into an uneasy bondage to each other. In the actual business of administration there is no question of today which Augustus had not to face and answer.

Source: Vorkosigan Saga, Cryoburn (2010), Chapter 13 (p. 257)

As quoted in Pioneers of Modern China : Understanding the Inscrutable Chinese (2005) by Khoon Choy Lee
Context: Nobody would say the cowshed was heaven and nobody would say the inhuman torture of so many victims be called a revolution of the proletariat. … A museum should be established to remind China of the follies and disasters that had fallen from 1966 to 1976. We cannot forget what had happened and history should not repeat itself.

Earthlight, p. 347
2000s and posthumous publications, The Collected Stories of Arthur C. Clarke (2001)