I posted that link just FYI.
I've read quite a lot about Duncan, and there's no doubt that she was a fraud.
Ghosthunter Harry Price took lots of photos of her with "ectoplasm" that is clearly cheesecloth draped around her (and if a ghosthunter says you're a fraud, then you're in trouble).
cj.23, you're quite right - she never made any kind of predictions, just did the normal medium/clairvoyant I'm-getting-a-Fred-or-maybe-a-Frederick stuff.
The seance that she was arrested for took place in Portsmouth (a big Navy town) and she claimed to have materialised a sailor from HMS Barham, a warship that had been sunk nine months previously, Her supporters have always said that this proved that she was a genuine medium, as the news of the sinking hadn't been released publicly. But in a Navy town it would have been difficult for quite a lot of people not to know something about it. Also, reports of the seance vary - witnesses seemed unable to agree on exactly what was said, or seen.
There is little doubt that Duncan was arrested because she was seen as a security risk - if she knew about the sinking of the Barham, somebody had to have been feeding her quite confidential information - who knew what else she could have blabbed about in the next seance?
For the last couple of years, there has been a sustained campaign by her family and supporters to have her pardoned; lots of rubbish has been circulated about her "amazing" powers as a result (she was supposed to have held private seances for Churchill and the Royals, for instance). I'm assuming that this "prediction" is yet another spurious claim, in the hope of getting more publicity for The Cause.
If you want to find out more, there's an excellent sceptical biography by Malcolm Gaskill, called Hellish Nell. (Don't mistake it for another Duncan biography of the same name, written by one of her granddaughters, and which is largely fable.)