A duplicate is any re-occurrence of an item in a collection.
Duplicate may refer to two or more identical strings in a list of values or strings, or multiple complex objects which are treated as the same object when compared with one with another. Duplicates often come in pairs but each one of a duo may be termed 'a duplicate'.
Use this tag for questions about locating, removing or otherwise dealing with duplicates.
Do not use this tag for questions relating to finding and otherwise dealing with duplicate files, for which there is [duplicate-files].