doppelganger (n.)

c. 1300, "twice as much or as large," also "repeated, occurring twice," also "of extra weight, thickness, size, or strength; of two layers," from Old French doble (10c.) "double, two-fold; two-faced, deceitful," from Latin duplus "twofold, twice as much," from duo "two" (from PIE root *dwo- "two") + -plus "more" (see -plus).

From early 14c. as "having a twofold character or relation," also "consisting of two in a set together; being a pair, coupled." From mid-14c. as "characterized by duplicity." The earliest recorded use in English is c. 1200, in double-feast "important Church festival."

Double-chinned is from late 14c.; double-jointed, of persons, is by 1828. Military double time (1833) originally was 130 steps per minute; double quick (adj.) "very quick, hurried" (1822) originally was military, "performed at double time."

The photographic double exposure is by 1872. The cinematic double feature is by 1916. Double figures "numbers that must be represented numerically by two figures" is by 1833. Double-vision is by 1714. Double indemnity in insurance is by 1832; double jeopardy is by 1817. The baseball double play is by 1866.

Double trouble "twice the trouble" is by 1520s; in 19c. America it was the name of a characteristic step of a rustic dance or breakdown, derived from slave dancing on plantations. A double-dip (n.) originally was an ice-cream cone made with two scoops (1936); the figurative sense is by 1940. Double bed "bed made to sleep two persons" is by 1779. Double life "a sustaining of two different characters in life" (typically one virtuous or respectable, the other not) is by 1888.

from Old English gang "a going, journey, way, passage," and Old Norse gangr "a group of men, a set," both from Proto-Germanic *gangaz (source also of Old Saxon, Old Frisian, Danish, Dutch, Old High German, German gang, Old Norse gangr, Gothic gagg "act of going"), of uncertain origin, perhaps from PIE root *ghengh- "to step" (source also of Sanskrit jangha "shank," Avestan zanga- "ankle," Lithuanian žengiu "I stride"). Not considered to be related to go.

The sense evolution is probably via meaning "a set of articles that usually are taken together in going" (mid-14c.), especially a set of tools used on the same job. By 1620s this had been extended in nautical speech to mean "a company of workmen," and by 1630s the word was being used, with disapproving overtones, for "any band of persons traveling together," then "a criminal gang or company" (gang of thieves, gang of roughs, etc.). By 1855 gang was being used in the sense "group of criminal or mischievous boys in a city." In American English, especially of slaves working on plantations (1724). Also formerly used of animal herds or flocks (17c.-19c.). Gangway preserves the original sense of the word, as does gang-plank.