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The 'You And I' singer passed the milestone in the third week of September, four months after the record was originally released.
Just over two million copies were purchased in the album's first week on sale, while 507,000 were bought the week after, reports Digital Spy.
The figure takes her total combined worldwide sales of all albums and repackages to approximately 16.7 million.
It is expected that the 25-year-old will finish 2011 with the second best-selling album of the year, behind Adele's 21, which is predicted to rack up 13 million copies by Christmas.