A haplography (from Greek ἁπλοῦς ‘single’ and γράφω ’to write’) is the writing of a segment of text once which appears twice (or more times) in the exemplar, e.g. defendum instead of defendendum. When a larger section of a text is left out, this is usually referred to as an omission, which may result in a lacuna.
The opposite, when what should be written once is written twice, is called dittography.
References
– Havet, Louis. 1911. Manuel de critique verbale appliquée aux textes latins. Paris: Hachette.
– Reynolds, Leighton Durham, and Nigel G. Wilson. 1974. Scribes and Scholars: A Guide to the Transmission of Greek and Latin Literature. 2nd ed. Oxford: Clarendon Press. || See pp. 204–205.
– West, Martin L. 1973. Textual Criticism and Editorial Technique Applicable to Greek and Latin Texts. Stuttgart: Teubner. || See pp. 24, 139.