Police are warning traders to beware of forged bank notes which have turned up in the South Norfolk town.
The fake twenties are copies of Scottish notes, which all have the same serial number.
One officer tweeted: 'Business owners - please be aware in the Diss area yesterday a male was trying to pay for goods with fake £20.00 notes. The fake £20.00 notes from Diss were Scottish notes and all have the same serial number.'
The Royal Bank of Scotland issues guidance on how to spot fakes on its website.
It says: 'Genuine notes have unique serial numbers therefore if you have two notes displaying the same serial number at least one of them is a counterfeit.'
It adds genuine notes also have a metallic thread embedded in the paper and when the note is held up to the light it appears as a bold continuous line. Raised print is used in some of the features on genuine banknotes and should feel slightly rough to the touch.
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