This is a navy trench coat by Ted Baker worn by the Duchess both before and after her wedding with Prince William.
It is a double breasted button front navy cotton coat with a chelsea collar and belted waist. There is an inverted pleat at the back. Here are both the navy and the cream versions in these pictures.
Navy looks really good on the Duchess in my opinion and she looks both classy and fresh especially in the pictures taken in Canada below.
1- Kate Middleton wore it on 14th March 2008 in Cheltenham to attend the racecourse.
2- The trench coat made another appearance on 4th July 2011 when the Duke and Duchess of Cambridge landed at Yellowknife, Prince Edward Island, Canada under the rain. It was worn over a dress and with navy stilettos.
3- The Duchess wore the same trench coat later that day 4th July 2011 at Charlottestown, Prince Edward Island, Canada to watch Prince William taking part to a helicopter demonstration. This time it was worn with navy jeans and wedges and also flat pumps for more comfort.