A composition mistake resulting in the same word, phrase, or entire line being set twice in a row. It often happens when a typesetter loses their place in the copy or during digital copy-pasting. Proofreaders mark these for deletion to prevent the repetition from reaching the press.
Heard on the job
The proofreader caught a doublet in the third paragraph where 'the the' slipped past layout.
Insider tip
Proofreaders often read text backward specifically to catch doublets and typos that the brain naturally glosses over.