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.
This the common law does not allow, because it produces useless prolixity, and always tends to confusion and to the multiplication of issues.
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 or of the same nature, to enforce only a single right of recovery.
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 to pleas perpetual or peremptory, and not to pleas dilatory; for in their time and place a man may use divers of them. But by this is not meant that any dilatory plea way be double, or, in other words, that it may 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 them successively, (each being in itself single,) in their proper order.
The inconveniences which were felt in consequence of this strictness were remedied by statutes and rules which provide, e.g., that 'it shall be lawful for any defendant in any action or suit, 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.' Such provisions are in force in most of the states.
Under such provisions, the defendant may plead as many different pleas in bar, (each being a single,) as he may think proper; but although this 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.
This extends only to pleas to the declaration, and does not embrace replications, rejoinders, nor any of the subsequent pleadings.