It is a matter, originally, of hypercorrection-- of some people thinking that the 't', because it is present, must be pronounced. Now, both pronunciations are accepted. Here is what the American Heritage Dictionary has to say: During the 15th century English experienced a widespread loss of certain consonant sounds within consonant clusters, as the (d) in handsome and handkerchief, the (p) in consumption and raspberry, and the (t) in chestnut and often.
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