Splitting Heirs

In this story written by Eric Idle of Monty Python fame, Tommy Patel (played by Idle) was an average man living in West London with a Hindi family. An American by the name of Henry Martin (Bullock; played by Rick Moranis) visits from New York and he and Tommy have a night on the town, leading to Tommy's firing.

When the 14th Duke dies from falling off of his yacht, brought upon by the family curse, Henry becomes the 15th Duke and hires Tommy back, but after attending the former 14th Duke's funeral, Tommy learns from Mrs. Patel that he was adopted after accidentally being left behind in a restaurant as a baby, and that he's the real Duke!

Tommy searches for evidence to prove his birthright and has an affair with a beautiful model named Kitty (Catherine Zeta Jones). When Tommy finds out that Kitty is Henry's fiancée, Tommy attempts to do away with Henry in order to claim his rightful position as the 15th Duke, causing hilarity to ensue in the process!

Tropes used in Splitting Heirs include:

Cloudcuckoolander: Henry might have shades of this, considering that he is often seen on roller-skates, and he wasn't aware of Tommy's attempts at killing him.
Ethical Slut: Henry's fiancée (and later in the movie, wife) Kitty, as she says that "sex before marriage is exciting" after having an affair with Tommy. Since Kitty's aware that Tommy was trying to kill Henry, she tells Tommy that she's pregnant with his (actually Henry's) baby to keep her husband safe.
Everything's Louder with Bagpipes: On the train back to England from France, Tommy plays La Marseillaise on the bagpipes.
Head-Tiltingly Kinky: Henry's reaction to the striptease dance before his and Kitty's wedding.
Helium Speech: On Tommy's 4th on-screen attempt to get Henry out of the way, he puts helium in Henry's scuba tank, making the latter speak in a squeaky voice.
Karma Houdini: Shadgrind (John Cleese), who causes the deaths of several bystanders in an attempt to bump Henry off almost gets his comeuppance when an escaped cougar lunges at him as he's escaping. However before he can be maimed, he is inexplicably saved by some French bicyclists in an upside down car. The epilogue then states how he became a Hollywood producer.
My God, What Have I Done?: When Tommy believes he's finally killed Henry, but he hasn't, because it was All Just a Dream. However, in actuality, Mrs. Bullock, the chef and Henry's biological mother, dies from being frozen.
Oblivious Adoption
Really Gets Around: Tommy's birth mother shows interest in any man, including her own son, a la Back to The Future , but until the truth comes out near the end of the movie.
Unfortunate Names / Who Names Their Kid "Dude"?: Tommy's birth name is Thomas Henry Butterfly Rainbow Peace Martin. Keep in mind he was born in The Sixties.
Would Hurt a Child: After Henry and Kitty's infant son is born, Shadgrind places a bomb in the baby carriage in an attempt to kill the child, but it killed the nanny instead.