These adjectives refer to copies that are replicas of one another or reproductions of an original. Duplicate designates a copy that is just like another or others. It may indicate two corresponding copies, one of which is a second or extra copy: duplicate receipts, the original retained by the bank, the carbon returned to the depositor. Or it may apply to multiple copies that look exactly alike: a dozen duplicate prints of a photograph. A duplicate copy may be made with or from an original, as by use of carbon paper or a duplicating machine; or it may be made from a pattern, as a stencil, negative or engraved metal plate: duplicate copies of a letter; duplicate copies of a book. At one extreme, duplicate designates a replica that is virtually indistinguishable from the original: a duplicate key. At the other extreme, it may indicate simply an accurate copy made at the same time or in the same form as the first: duplicate copy of an income-tax return, kept for the taxpayer’s personal records.

Like a duplicate copy, an exact copy or true copy is made from an original. But exact and true stress strict substantive accuracy and may or may not indicate correspondence in form. Exact implies a precise reproduction of all details in a standard or model.

• Writing painstakingly in longhand, the student made an exact copy of the printed poem; By means of the photocopying process, one can make an exact reproduction of the printed page.

True indicates absolute accuracy in reproduction, conformity to fact, and consequent validity. It stresses content rather than form.

• The school bursar certified the transcript as a true copy of the student’s record; The Registrar-General certified that the birth certificate was a true copy of facts recorded on the birth record.

Identical is applied to copies that are just like one another or that seem to be exactly alike. It is the strongest of these words and implies correspondence in every detail.

• So far as an untrained eye could tell, the reproduction was identical to the designer’s original.

Where a duplicate copy is clearly secondary, though it may be exact , identical implies equality, indicating mutual likeness: two women wearing identical dresses; identical prints made from a woodblock; Your pen is identical with mine. In plane geometry, congruent refers to figures that are identical in shape and size, filling exactly the same space. Triangles are congruent if every point of one can be brought into correspondence with every point of the other in space, so that the flat figures would coincide exactly if they were superimposed.