The plot sounds familiar because...

... L'Emmerdeur (2008) is a remake of the 1973 film L'Emmerdeur by Edouard Molinaro, which in the meantime became Billy Wilder's Buddy Buddy (1981), the screenplay for which was written by Francis Veber.

In other words, if you have already seen Buddy Buddy, the plot of L'Emmerdeur (2008) sounds familiar because the same person, Veber, wrote screenplays for both and also directed the latter. And for ten bonus points, the original play on which all three films were based was written by...?

You guessed it.

Hope that's all clear, Liebchen. Now stop reading this rubbish and go and watch L'Emmerdeur (1973). It's a great film.