From the A.Word.A.Day mailing list:
haplography (hap-LOG-ruh-fee) noun

The accidental omission of a letter or letter group that should be repeated in writing, for example, "mispell" for "misspell".

{From Greek haplo- single + -graphy writing.}

"In the apparatus of Trounce's edition, dittography occurs at line 266, haplography at line 352, and there are numerous erasures and corrections within the text."
Elaine Treharne, "Romanticizing the Past in the Middle English Athelston," The Review of English Studies, February 1999.

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