The Collaborative International Dictionary of English v.0.48:

Dupe \Dupe\ (d[=u]p), n. [F., prob. from Prov. F. dupe, dube; of
   unknown origin; equiv. to F. huppe hoopoe, a foolish bird,
   easily caught. Cf. Armor. houp['e]rik hoopoe, a man easily
   deceived. Cf. also Gull, Booby.]
   One who has been deceived or who is easily deceived; a gull;
   as, the dupe of a schemer.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dupe \Dupe\, v. t. [imp. & p. p. Duped (d[=u]pt); p. pr. & vb.
   n. Duping.] [Cf. F. duper, fr. dupe. See Dupe, n.]
   To deceive; to trick; to mislead by imposing on one's
   credulity; to gull; as, dupe one by flattery.
   [1913 Webster]

         Ne'er have I duped him with base counterfeits. --
                                                  Coleridge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dupe
    n 1: a person who is tricked or swindled [syn: victim, dupe]
    v 1: fool or hoax; "The immigrant was duped because he trusted
         everyone"; "You can't fool me!" [syn: gull, dupe,
         slang, befool, cod, fool, put on, take in, put
         one over, put one across]

Moby Thesaurus II by Grady Ward, 1.0:

250 Moby Thesaurus words for "dupe":
   Charlie McCarthy, Federal, Photostat, Xerox, agent, amanuensis,
   ancilla, apple-polisher, appliance, ass-licker, babe,
   backscratcher, backslapper, baffle, baggage agent, bamboozle,
   befool, beguile, betray, bilk, bluff, boob, bootlick, bootlicker,
   brown-nose, brownie, business agent, butt, byword,
   byword of reproach, cajole, catch, cheat, cheat on, chicane, child,
   child of nature, chump, circumvent, claim agent, clawback, clerk,
   clone, commercial agent, commission agent, con, conjure, consignee,
   contrivance, copy, counterpart, courtier, cozen, creature, cringer,
   customer agent, deceive, defraud, delude, derision, device, diddle,
   ditto, double, double-cross, dove, dummy, duplicate, duplication,
   dust, facsimile, factor, fair game, fall guy, fawner, fed,
   federal agent, figure of fun, flatterer, flimflam, flunky, fool,
   footlicker, forestall, freight agent, functionary, game, gammon,
   gazingstock, geminate, general agent, get around, go-between, goat,
   groveler, gudgeon, gull, handmaid, handmaiden, handshaker,
   hectograph, helot, hick, hoax, hocus-pocus, hoodwink, hornswaggle,
   humbug, implement, infant, infatuate, ingeminate, ingenue,
   innocent, instrument, insurance agent, interagent, intermediary,
   intermediate, intermedium, jackal, jest, jestingstock, job, joke,
   juggle, kid, kowtower, lackey, lamb, land agent, laughingstock,
   law agent, led captain, let down, lever, lickspit, lickspittle,
   literary agent, loan agent, lout, manifold, mark, mealymouth,
   mechanism, mediator, medium, mere child, microcopy, microfilm,
   midwife, mimeo, mimeograph, minion, mislead, mock, mockery, model,
   monkey, mug, multigraph, multiply by two, news agent, noble savage,
   oaf, official, organ, outmaneuver, outreach, outsmart, outwit,
   overreach, parliamentary agent, passenger agent, patsy, pawn, peon,
   pigeon, play one false, plaything, press agent, puppet,
   purchasing agent, pushover, put on, put something over,
   quadruplicate, real estate agent, redouble, reduplicate, repeat,
   repetition, replica, replicate, replication, representation,
   reproduce, reproduction, rip off, rook, rube, sales agent, sap,
   secretary, serf, servant, simple soul, slave, snooker, snow,
   spaniel, special agent, stat, station agent, steward, stock,
   stooge, string along, stultify, suck, sucker, swindle, sycophant,
   take, take in, target, theatrical agent, ticket agent, timeserver,
   toad, toady, tool, toy, trace, transcribe, travel agent, trick,
   triplicate, truckler, trusting soul, tufthunter, twin, two-time,
   unsophisticate, vehicle, victim, victimize, walking delegate,
   yes-man, yokel