Retailers are being pressed to create customer databases to avoid the wrath of teenage girls who turn up to school balls wearing the same dress as someone else.
The popular students are even dictating what colour gowns their friends can wear and Facebook pages have been created where girls post photos of their dresses to warn their peers off buying similar garb.
Hellen Burley of Affordaball Dresses in Mt Roskill, Auckland, said she had seen tears and tantrums from both students and their mothers.
"We try very, very hard to only sell one dress per school. We keep a register and we update that register all the time, so we write down the dress number, the colour, everything like that." The system didn't always work.
"One girl told us that her friend said she could wear the same dress in a different colour, so we sold it to her. Then we had the mother of the other girl on the phone saying, 'It's not okay'. She was very, very angry. So then I rang up the other girl and she said, 'Well, you're not the dress police', and she wasn't prepared to change it. It is so dramatic and it happens a lot.
"They are like bridezillas, it is nearly like a wedding. Girls are so quick to turn on each other, it's horrible.
"I've had girls come in and say, 'Oh no, I'm not allowed to wear that colour, I have to wear white'. Obviously the kingpin has dictated to her friends what colour they can wear."
On another occasion, a student's mother was left in tears after her daughter came home and told her they had to return her dress because another girl had decided it was "too similar" to her own.
"I had the mother coming in bawling her eyes out. She was so upset and her daughter was so upset. I was just thinking, 'How can girls this young have so much control over their friends?' It's amazing."
Parents were often spending $1000 or more on the ball experience, Ms Burley said.
Mount Albert Grammar School student Negin Shademan, 17, tried on a replica of the strapless number Angelina Jolie wore to the Academy Awards in February - one of the most popular designs this ball season - at Ms Burley's store this week.
Asked how she would feel if someone else turned up to the event on June 30 with the same dress, she said: "It would be a dream-shattering moment. You'd constantly be wondering if she looked better in it. You talk about what kind of dress you're getting ages before the ball and make sure everyone knows."
Social anthropologist Dr Donna Swift said teenagers were imitating female behaviour in movies and television and social networking sites had increased competitiveness.
"If you look at Desperate Housewives or the reality shows, what they feature is the bitchiness, the backstabbing, and they highlight that as normalised behaviour."