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It is not true, the Titanium case design lasted two years, the Aluminium case design lasted five and a half years and the current case design is just shy of being two and a half years old.
Proof: www.everymac.com or any other site chronicling Apple's models, which could be found via some search engine, like Lycos or AltaVista or Bing or Yahoo or MetaCrwaler or ... .
 
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