Edit: The 5th edition of Freckelton’s Indictable Offences in Victoria is now out. A splash about it cam be found here.
A charge is bad for duplicity if there is more than one offence alleged in the one charge.
The law doesn’t allow duplicity because an accused person has the right to know what is alleged against them. A duplicitous charge is not a nullity (though some would like to think so!). The remedy to duplicity is to require the prosecution to elect the charge it wants to proceed with. If the prosecutor refuses to elect then the court has discretion to dismiss the charge: Walsh v Tattersall (1996) 188 CLR 177.
In Rixon v Thompson  VSCA 84, the accused was convicted in the Magistrates’ Court of one charge of indecent assault. The prosecution case alleged several incidents. Each incident could have been the subject of its own charge. The accused unsuccessfully appealed to the Supreme Court, and then to the Court of Appeal.
Several points came out of the combined judgment of Maxwell P, Weinberg JA and Kyrou AJA:
- deciding upon duplicity is not a discretionary decision. Once a court considers a charge duplicitous, the law obliges the court to require the prosecution to elect the charge to proceed
- the prohibition on duplicity is as strict in the Magistrates’ Court as in any other court
- duplicity isn’t decided by considering only if there’s unfairness to the accused. Even if the accused is provided a detailed statement specifying the acts alleged, if the charge is ‘patently duplicitous’, the prosecution must be required to elect
- but, there are exceptions to the rule against duplicity, among them the single (or composite) transaction analysis outlined by Ashley JA in R v Heaney  VSCA 74
In Heaney‘s case, a conviction for stabbing was challenged as duplicitous. Heaney was also charged with one count of attempted murder. The evidence alleged two thrusts with the knife. The (differently composed) Court of Appeal rejected counsel’s submission that the jury’s verdict was bad for duplicity, at  – .
If you have access to Ian Freckelton’s Indictable Offences Victoria, I also recommend his helpful commentary about duplicity and the presentment rules. He covers scenarios (from memory) such as: five punches in quick succession — one assault, or five? A man steals 4 bags of wheat, one at a time, over half an hour — one theft or four? (Sadly, the Fourth Edition is out of print, and the Fifth Edition is still pending.)
To come back to Rixon v Thompson, in the end the Court of Appeal decided the single charge was not duplicitous. On one view, there were multiple discrete offences. Alternatively, there was one continuous criminal episode over several hours. Viewed that way, with the particulars provided to the accused, the Court considered the Heaney exception of ‘single transaction analysis’ applied. That meant the charge was valid, and Rixon’s appeal was dismissed.