The Collaborative International Dictionary of English v.0.48:

duplicity \du*plic"i*ty\, n.; pl. duplicities. [F.
   duplicit['e], L. duplicitas, fr. duplex double. See
   Duplex.]
   1. Doubleness; a twofold state. [Archaic]
      [1913 Webster]

            Do not affect duplicities nor triplicities, nor any
            certain number of parts in your division of things.
                                                  --I. Watts.
      [1913 Webster]

   2. Doubleness of heart or speech; insincerity; a sustained
      form of deception which consists in entertaining or
      pretending to entertain one set of feelings, and acting as
      if influenced by another; bad faith.
      [1913 Webster]

            Far from the duplicity wickedly charged on him, he
            acted his part with alacrity and resolution.
                                                  --Burke.
      [1913 Webster]

   3. (Law)
      (a) The use of two or more distinct allegations or
          answers, where one is sufficient. --Blackstone.
      (b) In indictments, the union of two incompatible
          offenses. --Wharton.

   Syn: Double dealing; dissimulation; deceit; guile; deception;
        falsehood.
        [1913 Webster]

WordNet (r) 3.0 (2006):

duplicity
    n 1: a fraudulent or duplicitous representation [syn:
         fraudulence, duplicity]
    2: acting in bad faith; deception by pretending to entertain one
       set of intentions while acting under the influence of another
       [syn: duplicity, double-dealing]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "duplicity":
   Janus, Machiavellianism, ambidexterity, ambiguity, ambivalence,
   artfulness, artifice, bad faith, biformity, bifurcation,
   conjugation, craft, cunning, deceit, deceitfulness, dichotomy,
   dirty pool, dirty trick, dirty work, dishonesty, dissemblance,
   dissimulation, double-dealing, doubleness, doubleness of heart,
   doublethink, doubling, dualism, duality, duplexity, duplication,
   equivocality, faithlessness, falseheartedness, falseness,
   foul play, furtiveness, guile, halving, hypocrisy, improbity,
   indirection, insidiousness, irony, low cunning, pairing,
   perfidiousness, perfidy, polarity, shiftiness, sneak attack,
   sneakiness, surreptitiousness, treacherousness, treachery,
   twinning, two-facedness, twoness, underhandedness, wile



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DUPLICITY, pleading. Duplicity of pleading consists in multiplicity of
distinct matter to one and the same thing, whereunto several answers are
required. Duplicity may occur in one and the same pleading. Double pleading
consists in alleging, for one single purpose or object, two or more distinct
grounds of defence, when one of them would be as effectual in law, as both
or all.
     2. This the common law does not allow, because it produces useless
prolixity, and always tends to confusion, and to the multiplication of
issues. Co. Litt. 304, a; Finch's Law, 393.; 3 Bl. Com. 311; Bac. Ab. Pleas,
K 1.
     3. Duplicity may be in the declaration, or the subsequent proceedings:
Duplicity in the declaration consists in joining, in one and the same count,
different grounds of action, of different natures, Cro. Car. 20; or of the
same nature, 2 Co. 4 a; 1 Saund. 58, n. 1; 2 Ventr. 198; Steph. Pl. 266; to
enforce only a single right of recovery.
     4. This is a fault in pleading, only because it tends to useless
prolixity and confusion, and is, therefore, only a fault in form. The rule
forbidding double pleading "extends," according to Lord Coke, "to pleas
perpetual or peremptory, and not to pleas dilatory; for in their time and
place a man may use divers of them." Co. Litt. 304, a. But by this is not
meant that any dilatory plea way be double, or, in other words, that it way
consist of different matters, or answers to one and the same thing; but
merely that, as there are several kinds or classes of dilatory pleas, having
distinct offices or effects, a defendant may use "divers of them"
successively, (each being in itself single,) in their proper order. Steph.
Pl. App. note 56.
     5. The inconveniences which were felt in consequence of this strictness
were remedied by the statute, 4 Ann. c. 16, s. 4, which provides, that " it
shall be lawful for any defendant, or tenant, in any action or suit, or for
any plaintiff in replevin, in any court of record, with leave of the court
to plead as many several matters thereto as he shall think necessary for his
defence."
     6. This provision, or a similar one, is in force, probably, in most of
the states of the American Union.
     7. Under this statute, the defendant may, with leave of court, plead as
many different pleas in bar, (each being a single,) as he may think proper;
but although this statute allows the defendant to plead several distinct and
substantive matters of defence, in several distinct pleas, to the whole, or
one and the same part of the plaintiff's demand; yet, it does not authorize
him to allege more than one, ground of defence in one plea. Each plea must
still be single, as by the rules of the common law. Lawes, Pl. 131; 1 Chit.
Pl. 512.
     8. This statute extends only to pleas to the declaration, and does not
embrace replications, rejoinders, nor any of the subsequent pleadings.
Lawes, Pl. 132; 2 chit. Pl. 421; Com. Dig. Pleader, E 2; Story's Pl. 72, 76;
5 Am. Jur. 260-288. Vide) generally, 1 Chit. Pl. 230, 512; Steph. Pl. c. 2,
s. 3, rule 1; Gould on Pl. c. 8, p. 1; Archb. Civ. Pl. 191; Doct. Pl. 222; 5
John. 240; 8 Vin. Ab. 183; U. S. Dig. Pleading, II. e and f.