Duplicity is a legal term used to describe the error committed when the charge (known as a count) on an indictment describes two different offences. An indictment may contain more than one count, but each count must allege only one offence, so that the defendant (and the jury) can know precisely what offences he is accused of. If a count is poorly drafted so that it alleges two offences, it is said to be duplicitous. A duplicitous count is defective and must be quashed by the judge, unless the judge permits the count to be amended so that it only alleges one offence, or is split into two counts. If a duplicitous count is not noticed until after the defendant has been convicted on it, the verdict may well be void.
This is a completely different situation from when two different counts each allege the same offence, which is sometimes wrongly referred to as duplicity. This may be due to confusing the word duplicity with duplicate.
One of the best known judgments in Australia relating to duplicity is Johnson v Miller[1] which is still referred to as a sound principle of law. A number of WorkCover prosecutions in South Australia have been found to be duplicitous and offer substantial insight into the issue in some of the case law which has evolved on the subject. Such a case is Walsh v Tattersall.[2]