Definition - A letter or word that is incorrectly repeated by a copyist who is making a written copy of a text.

Example -
When making the first written copy of the Ten Commandments, the scribe accidentally writes: Thou shalt not not kill.

Etymology -
The term derives from a Greek word meaning "double writing."

Oxford English Dictionary -
Its first citation is from 1874:
"The letters in italics stand, probably, for probeidem, i.e. a dittograph for probe and pridem."
(T. H. Key Language 407 note)

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