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Two unrelated young girls who happen to look identical suddenly meet. Amanda Lemmon (Mary-Kate Olsen) is an orphan, and she is about to be adopted by a family whom she doesn't like, the Butkises. She actually wants her child-loving case worker Diane Barrows (Kirstie Alley) to adopt her instead. Diane would like to do the same thing, but authorities will not let her because of her low salary. Alyssa Callaway (Ashley Olsen) is coming home from her boarding school's piano recital competition, only to find that her wealthy father Roger (Steve Guttenberg) is about to marry Clarice Kensington (Jane Sibbett) a socialite who (secretly) [[ChildHater hates kids]] and baseball (which Roger adores).

The girls switch places and find out that Diane and Roger would fit together perfectly. So they arrange "chance" meetings with the desired result: they fall in love with each other. After some turbulences, Alyssa (who poses as Amanda) ends up being adopted by the Butkises. She and Diane (while looking for Alyssa) find out the only reason they have adopted many kids was for them to work in their salvage yard.

When Clarice secretly spies on Roger and Diane, she decides to move up the wedding from the next month to the next day. After Amanda, who poses as Alyssa, proves to the family butler Vincenzo (Philip Bosco) who she really is at the church, he summons to have the real Alyssa picked up from the Butkises' salvage yard to stall the wedding. Once she and Diane end up at the church, Roger stops the wedding and tells Clarice that he fell in love with Diane. Furious, she slaps him and prepares to do the same to both Amanda and Alyssa but is stopped by both Vincenzo and Diane. She storms out of the church, embarrassed. Alyssa embarrasses Clarice even more by stepping on her dress, revealing the back of her underpants in front of everybody, including people with cameras. Roger and Diane both find out in the end that it was Amanda and Alyssa that arranged all the meetings between the two of them the entire time.
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!!This film contains examples of:
* AddedAlliterativeAppeal: Camp Calloway.
* AllGirlsLikePonies: Alyssa.
* AlliterativeName: The Butkises change the names of all the children they adopt to a name that starts with "B", resulting in this. [[spoiler:Alyssa was called Betty during her time there.]]
* AlmostKiss: Happens a couple of times between Roger and Diane.
* AltarTheSpeed: Clarice attempts this after noticing the growing bond between Roger and Diane.
* BigOMG: After Clarice sees that Amanda spit gum into her hair.
* BitchInSheepsClothing: Clarice.
* BigDamnHeroes: [[spoiler: The people from the Calloway estate who show up in a ''helicopter'' to rescue Alyssa, who had been posing as Amanda, from the Butkises' salvage yard.]]
* BlondesAreEvil: Clarice again.
* ChekhovsSkill: Alyssa's talent at miming passing out first gets used when she finds out that Clarice and Roger are getting married. It gets used again during a game of Charades.
* DepartmentOfChildDisservices: Diane's employers don't let her adopt Amanda because she doesn't make enough money, yet are perfectly willing to foist Amanda on the Butkises - a family that turns out to have been adopting kids by the half dozen to force them to work in their junkyard] The fact that a neighbor brings this last point up in casual conversation makes it even worse - the Butkises aren't even keeping it a secret.
* FamilyThemeNaming: The adopted Butkises all have B names.
* {{Fainting}}: Alyssa's (fake) reaction when her father reveals that he intends to marry Clarice.
* FullHouseMusic: When Diane talks Amanda into going to summer camp.
* GoldDigger: Take a wild guess...
* HeroStoleMyBike: In the climax, [[spoiler:Diane and Alyssa steal a man's horse-drawn buggy in order to get to the chapel in time to stop the wedding.]]
* HowManyFingers: After Roger falls off of his horse.
* {{Hypocrite}}: Roger hates cell phones, but he made his fortune by investing in the cell phone industry.
* IAteWhat: Amanda with escargot at a dinner party.
* IdenticalStranger: Alyssa and Amanda.
* IKnowWhatWeCanDoCut: The girls' decision to get Roger and Diane to meet while horseback riding.
* MamaBear: Diane, particularly near the end of the film.
* MatchCut: When Amanda gets settled in at camp, she places her hat on her bed. The scene cuts to Alyssa doing the same in her room.
* NakedFreakOut: Happens to Clarice after [[spoiler:Alyssa steps on her wedding dress, ripping off the entire skirt.]]
* OffToBoardingSchool: Clarice plans to send Alyssa to a boarding school in Tibet.
* {{Overcrank}}: To avoid being discovered, Alyssa trips Frankie, causing him to cover Roger in macaroni and cheese. It triggers a massive FoodFight.
* PapaWolf: Vincenzo, like Diane, gets a moment of this near the end of the film.
* ParentWithNewParamour: She's terrible.
* RaceForYourLove [=/=] SpeakNowOrForeverHoldYourPeace [=/=] WeddingSmashers: The climax.
* RescueRomance: Roger saves Diane from her horse, and Diane gives Roger first aid when he falls off of his horse.
* SelfMadeMan: Roger Calloway. He made his money ''"the old-fashioned way - pure dumb luck"''
* ServileSnarker: Vincenzo.
* SocietyMarchesOn: Central Park's horse-drawn carriages are at the center of an animal rights debate in RealLife. Fortunately, the horse in the climax gets to retire.
* TomboyAndGirlyGirl: The film almost gleefully beats you over the head with just how much Amanda and Alyssa are this, respectively. Amanda's tomboyishness is even stated to be a main reason she's having trouble getting adopted.
* UptownGirl: Diane believed rich guys like Roger would never marry people like her.
* [[WhatDoesSheSeeInHim What Does He See In Her:]] Clarice is a ChildHater sports hater bitch. Yet Roger is marrying her.
* WitchWithACapitalB: Clarice.
* WickedStepmother: Clarice is aiming to become one to Alyssa.
* WouldHurtAChild: When Clarice realizes she's lost, she is only too ready to hit the girls. Only timely interference from Vincenzo and Diane keep her from doing so.
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