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Possible Duplicate:
When did it become correct to add an 's' to a singular possessive already ending in 's'?

I learned from school to use Dawkins', for example

That is Mr. Dawkins' house.

But I see many people write Dawkins's instead. Is it something related to American English as opposed to British conventions?

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Both forms are valid, depending on the style guide you follow. It does not appear to be a regional thing. Wikipedia has an overview.