When is a 'unique' visitor, unique?

Jemima Kiss at The Guardian has sparked off a sharp little debate about website visitor numbers and the popularity of political blogs.

She claims that Guido Fawkes and Iain Dale, two of the more popular political bloggers in the UK, have inflated the number of visitors they get to their websites by inadvertently misrepresenting their 'unique visitor' statistics.

Both Guido Fawkes and Iain Dale appear in the comments section denying the allegation.