Nouns

(n)extra, duplicatesomething additional of the same kind“he always carried extras in case of an emergency”
(n)duplicate, duplicationa copy that corresponds to an original exactly“he made a duplicate for the files”

Verbs

(v)duplicate, reduplicate, double, repeat, replicatemake or do or perform again“He could never replicate his brilliant performance of the magic trick”
(v)twin, duplicate, parallelduplicate or match“The polished surface twinned his face and chest in reverse”
(v)duplicatemake a duplicate or duplicates of“Could you please duplicate this letter for me?”
(v)double, duplicateincrease twofold“The population doubled within 50 years”

Adjectives

(s)duplicateidentically copied from an original“a duplicate key”
(s)duplicate, matching, twin, twinnedbeing two identical