“Scientists ignorant of history are not so much condemned to repeat it, as to be confused and unenterprising.”

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

Source: The Intelligent Investor

This famous statement has produced many paraphrases and variants:
Those who cannot learn from history are doomed to repeat it.
Those who do not remember their past are condemned to repeat their mistakes.
Those who do not read history are doomed to repeat it.
Those who fail to learn from the mistakes of their predecessors are destined to repeat them.
Those who do not know history's mistakes are doomed to repeat them.
There is a similar quote by Edmund Burke (in Revolution in France) that often leads to misattribution: "People will not look forward to posterity, who never look backward to their ancestors."
The Life of Reason: The Phases of Human Progress (1905-1906), Vol. I, Reason in Common Sense
Source: The Life of Reason: Five Volumes in One
Context: Progress, far from consisting in change, depends on retentiveness. When change is absolute there remains no being to improve and no direction is set for possible improvement: and when experience is not retained, as among savages, infancy is perpetual. Those who cannot remember the past are condemned to repeat it.

Letter to Thomas Jefferson (23 August 1787), The Works of John Adams.
1780s

Source: 1990s Lies My Teacher Told Me: Everything Your American History Textbook Got Wrong (1995), p. 1

Quasar, Quasar, Burning Bright (1978), p. 235
General sources

Letter to C. P. Scott (20 January 1920), in Trevor Wilson (ed.), The Political Diaries of C. P. Scott, 1911-1928 (London: Collins, 1970), p. 380
1920s

" 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)

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