Dittography
Dittography is the accidental, erroneous act of repeating a letter, word, phrase or combination of letters by a scribe or copyist. The term is used in the field of textual criticism. The opposite phenomenon, in which a copyist omits text by skipping from a word or phrase to a similar word or phrase further on, is known as haplography. Example
Papyrus 98 in Rev 1:13 has  instead of  (doubled μ). The Codex Vaticanus repeats the word  in John 13:14. the phrase "Great is Artemis of the Ephesians" appears twice in Acts 19:34 in the Codex Vaticanus, while it only appears once in other manuscripts. References

Proofreading
Textual criticism
Textual scholarship