Doublet

noun

1. A pair of two similar or equal things; couple.
2. One of two or more different words in a language derived from the same etymological root but having different phonological forms (e.g., toucher and toquer in French or shade and shadow in English).
3. In textual criticism, two different narrative accounts of the same actual event.
4. (lapidary) An imitation gem made of two pieces of glass or crystal with a layer of color between them.
5. A word or phrase set a second time by mistake.