Most "twist" movies are thrillers, aiming to end on a "WHOA!" "Duplicity" is definately a twist-movie, but it's content to end on an modestly-upbeat "Heh." In exchange for the lack of thrills, we get a lot of very talented actors (AND Julia Roberts... ahem...) exchanging witty espionage banter that the film hopes we'll find exponentially funnier through the magic of ironic juxtaposition, i.e. all the skullduggery is between rival cosmetics tycoons.
Roberts and Clive Owen (THE go-to-guy actor when the breakdown calls for "James Bond only not") are a pair of rival spies (formerly CIA and MI6, respectively) who meet-cute again (or do they?) on opposite sides (or are they?) of the hired counter-intel teams for two New York cosmetics barons. One of the CEOs (Tom Wilkinson) is sitting on a secret miracle product (or is he?) sought by his rival (Paul Giamatti.) The pair of spies, who had a prior romantic encounter years ago (or is it ongoing?) hatch a plan to double-cross both sides and make off with the Big Money themselves. The timeline cuts back and forth between the present-plan and the past of the two leads, aiming to keep the audience guessing as to who's been on who's side and for how long.
It's all suitably breezy and well paced, and it's doing it's damndest to recreate the "sophisticated" (read: "detached") couples-sparring that informed oldschool caper/romance flicks like "The Thomas Crowne Affair" or "Charade;" but in the end it's a house of cards stacked entirely too high for the flimsy material said cards are made of... though, it must be said, it MIGHT have helped to not hinge so much of the film on the concept of Julia Roberts as a source of potent sexual power. Nice effort, though.