1. Something consisting of two identical or similar part
2. One of a pair; specifically; one of two or more words (as guard and ward) in the same language derived by different routes of transmission from the same source

ETYM Old Fren. doublet, French doublet, dim. of double double. Related to Double.
Close-fitting jacket, with or without sleeves, worn by men. Derived from the gipon, which was a tight-fitting jacket with long sleeves and buttons down the front, the doublet formed part of everyday dress from the mid-15th century. In the 16th century, as fashions changed, the doublet was worn long to the knees, but with a parting at the front to reveal the codpiece. During the 18th century the length of the doublet shrank and it eventually evolved into the waistcoat.
A man's close-fitting jacket; worn during the Renaissance.