Holy shit, Duplicity blew me away. As far as I can recall just now, it's far and away the best "relationship movie" I have ever seen.
Most movies about couples fall into one of two categories. Many of them are about The Joys of Falling in Love--the guy meets the most wonderful woman, and it's the story of how he woos her, and how great it all is. Fuck that. I don't need to be told that falling in love is nice, and I won't be winning my partner in any way that resembles a cinematic narrative.
The other basic trope deals with How Precious is Love and How Painful Its Loss--they have the perfect relationship until he's sent to The Front, or she gets cancer, or what have you. That's a damn insult, and the whole genre from Moulin Rouge to La Traviota is the most cynical kind of pornography. It's just schadenfreude dressed up in the guise of "showing us how to grieve". Honestly, do people really think that I won't know what to do if the love of my life gets cancer? That I'll need a damned Hollywood movie to show me? It makes me sick.
As you might be able to notice, this is a subject that evokes strong feelings for me. At any rate, Duplicity is a movie about two people in an established relationship, and it shows them dealing with how to love each other and trust each other and hurt each other and forgive each other and all that. About two people who know all of each others faults and weaknesses, and who still love each other. This is something that seems worthy of exploration!
It doesn't go into too much detail (and the details are different for everyone, anyway) and it is also a tightly paced, brilliantly written spy thriller. It is also extremely funny. Tony Gilroy is great.