и так и так пишут

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Taken from an actual contract with a British company

Alternative 1: executed in three originals, having equal effect
Alternative 2: is being executed in three exemplars.
Alternative 3: has been executed in three exemplars.
-- "executed in three originals" gets the most Google hits, over 3000.
-- "executed in three duplicate originals" is redundant, and grates on the ear. It has only 577 Google hits.
"Exemplars" is an idiomatic term in U.S. English, used by some large law firms etc. However, it gives less than 10 Google hits. Therefore I would class it as idiosyncratic with law firms that I have worked for. On the other hand, it is closer to the author.
Translation of entire phrase: (Настоящий договор составлен в трех экземплярах, один из которых хранится в делах нотариуса XXX по адресу: __________, по экземпляру выдается XXX и YYY в лице ZZZ.)
The present Contract Document is being executed in three originals, one of which is being stored in the files of the Notary, XXX, at address ___; one original is being issued to XXX [sic -- viz. a party not the Notary]; and one original is being issued to YYY (through his/her/its representative, ZZZ).
Alternative of the entire phrase: The present Contract Document is being executed in three exemplars, one of which is being stored in the files of the Notary, XXX, at address ___; one exemplar is being issued to XXX [sic -- viz. a party not the Notary]; and one exemplar is being issued to YYY (through his/her/its representative, ZZZ).

По моему

This is a usual wording in a formal document