(1):

(v. t.) To divide into two by natural growth or spontaneous action; as, infusoria duplicate themselves.

(2):

(n.) That which exactly resembles or corresponds to something else; another, correspondent to the first; hence, a copy; a transcript; a counterpart.

(3):

(v. t.) To double; to fold; to render double.

(4):

(v. t.) To make a duplicate of (something); to make a copy or transcript of.

(5):

(n.) An original instrument repeated; a document which is the same as another in all essential particulars, and differing from a mere copy in having all the validity of an original.

(6):

(a.) Double; twofold.