LONDON (Reuters) - Auction houses pinching themselves after bumper 2011 sales are now turning their attention to 2012, amid cautious optimism that the two-year bull run for top works of art will continue. Sotheby's and Christie's, the world's leading auctioneers, hold a series of big sales in London over the next two weeks at which the old rivals expect to raise more than 500 million pounds ...

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