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[Photos: Keira in Erdem and Elie Saab S/S 2009. Rex Images. CLICK TO ENLARGE]

The trickle up / trickle down theory applies to fashion in so many ways. You have your obvious one; designer creates an item, and the high street or bargain retailers copy it. Then you have the reverse, when a trendy young things wears something and a designer spots it, becomes inspired and churns out a £600 version. But it's not so often you see a designer inspired by something their contemporaries have done a season before. Still, here's a prime example of it. On the left is Keira Knightley sporting Erdem's purple strapless frock with a frilled flower detail on the bandeau. On the right are two dresses from Elie Saab's recent Paris Fashion Week S/S 2009 show. Spot the difference?!