Dittography is the accidental, erroneous act of repeating a letter, word, phrase or combination of letters by a scribe or copyist.

Example

⁹⁸ in Rev 1:13 has περιεζωσμμενον instead of περιεζωσμενον (doubled μ). Codex Vaticanus

Codex Vaticanus

The Codex Vaticanus , is one of the oldest extant manuscripts of the Greek Bible , one of the four great uncial codices. The Codex is named for the residence in the Vatican Library, where it has been stored since at least the 15th century...

  in John 13:14 word διδασκαλος is repeated twice. In Codex Vaticanus in Acts

Acts of the Apostles

The Acts of the Apostles , usually referred to simply as Acts, is the fifth book of the New Testament; Acts outlines the history of the Apostolic Age...

, a book of the Bible

Bible

The Bible refers to any one of the collections of the primary religious texts of Judaism and Christianity. There is no common version of the Bible, as the individual books , their contents and their order vary among denominations...

, verse 19:34, the phrase "Great is Artemis of the Ephesians" appears twice while it only appears once in other manuscripts.