Two characters have a difficult relationship not because of their differences, but because of their similarities. This is most commonly seen between parents and children or romantic couples, but there's plenty of room for it in other relationships.
This can be a sign of latent (or not-so-latent) self-loathing; the character hates himself, so he also hates people who remind him of himself. Or it can be a matter of certain traits needing to be complemented rather than mirrored in a relationship: someone who talks all the time and someone who would rather listen are obviously better off with each other than with partners like themselves.
See Foil for a list of examples of how differing personalities can be better together. Compare Opposites Attract, You Remind Me of X, Hypocritical Humor, You Are What You Hate, Turn Out Like His Father, Evil Parents Want Good Kids, Your Approval Fills Me with Shame, Doppelgänger Dating. Contrast Birds of a Feather.
- In Brave10, Rokuro and Nanakuma are a pair of Tsundere identical twins with Undying Loyalty to their respective masters, but that only fuels more conflict between them.
- It's mentioned at some point that Asuka and Shinji of Neon Genesis Evangelion are something like this, which comes across as somewhat humorous considering she's the page image for Fiery Redhead and he's a neurotic Extreme Doormat. However, later on it's revealed that they actually have very similar backgrounds — both having the same internal struggles and abandonment issues — and the main difference in is how they handle their problems.
- Art Spiegelman, the author/narrator of Maus, mentions this off-hand to his fiancée, as he once had a girlfriend who was also Jewish and Middle-Class, but Art then mentions that because they were so similar that it was weird to get erotic with the girl.
- In one Betty and Veronica story, the titular girls bumped into two cute guys in the mall, Jon and Benny, who were pretty much their Spear Counterparts. By the end of the story, Betty and Veronica decided that they wouldn't make a good match because they have too much in common.
- In the Thor (2014) series, Lady Sif suggests this is why she and Thor never worked as a couple.
Thor: You speak your mind with the same fury with which you cross swords. 'Tis why I've always loved you, my lady.
Sif: Aye, and 'tis why our love has always ended so badly. I fear we are too much alike, you and I.
- Taken to an extreme in Calvin and Hobbes when Calvin completely clashes with his duplicate.
Hobbes: He's a duplicate of you, all right.
Calvin: What do you mean? This guy is a total jerk!
- Fight Club: This is why the main character doesn't like the Love Interest, Marla, at first.
- In Road to Perdition, Michael Sullivan, Jr., feels like The Unfavorite and eventually asks his father straight out if he liked Michael's murdered brother better. His father says that he didn't, and that if he treated them differently, it was because Michael reminded him of himself, and he didn't want his own son to Turn Out Like His Father.
- Avengers: Infinity War: Tony Stark and Stephen Strange share a strong mutual dislike of the other, primarily because they're both Insufferable Genius Deadpan Snarkers who are evenly matched in both capacities.
- In A Tree Grows in Brooklyn, Francie takes after her mother, Katie — striking her as ordinary — while her younger brother Neeley takes after their father, a handsome and loving but unreliable alcoholic. Katie devotes herself to making sure Neeley grows up to be like the man she fell in love with, but without his flaws, leaving Francie to fend for herself. She tries to hide her Parental Favoritism, but Francie understands her mother's motives instinctively.
- The Hunger Games: One of the reasons why Katniss never considers having a relationship with Gale Hawthorne: they are so alike that Katniss cannot see him as a romantic interest; not to mention everything that happened during the rebellion, particularly the fact that the bombs Gale designed killed Katniss' sister, Prim.
- It's also the reason she and Haymitch and have such a combative relationship—Peeta observes that they're far too much alike.
- Jo and Laurie from Little Women are one of the most famous examples, although when Jo turns down his proposal she also points out the things that are too different about them (she wouldn't like high society, and he wouldn't like her writing).
- Percy Jackson and Thalia Grace. As Annabeth says, "You guys are so alike it's scary. I mean, either you would've been best friends or you would've strangled each other."
- Maya and Frank in the Red Mars Trilogy are attracted to each other, but they're both such Manipulative Bastards that it turns into a miserable time for both of them.
- A key element of the novel The Stone Angel is that Hagar Shipley insists that her younger son John is just like her while his older brother Marvin is like their father. In reality, the opposite is true and Hagar is simply deceiving herself. The stodgy, proper Marvin is just like his mother which is probably why she dislikes him so much.
- In the Tortall Universe, Alanna's relationship with Liam doesn't work out because they're such similar people: stubborn and temperamental warriors with Chronic Hero Syndrome who aren't willing to adjust for each other, or for anything. George, the man she marries, is a much more laid-back, humorous type.
- On 30 Rock, Liz broke up with Carol for this reason, and later it was made explicit that she and Criss were a good fit because of their differences. There was also an episode where Liz and Jenna had a fight and went looking for new best friends more like themselves, only to realize why they needed each other: Jenna is an Attention Whore who can't be around other people who will compete for the spotlight, and Liz is so negative that having another negative person to bounce off is just "exhausting"; she needs someone so vacuous and self-absorbed that they'll just let her vent because they're not even listening. (This helps answer the oft-asked and oft-lampshaded question of how these two were ever friends to begin with.)
- On Babylon 5, this was the reason for John Sheridan's divorce from his first wife, Elizabeth Lochley. They had a lot in common, including their commanding dominant personalities. They quickly realized that getting married was a mistake, and claimed the divorce saved their friendship.
- In The Borgias, Rodrigo and Cesare's increasingly strained relationship is in no small part because they're so similar to each other; "all the fire and the fury, the drive, the pitiless ambition" which makes Cesare bridle at his constantly being denied real power, and Rodrigo's guilt for what they do gets exacerbated when he sees his own traits in Cesare. At the end of Season 2, Rodrigo starts to explain to Cesare that Rodrigo favoured Cesare's idiot younger brother Juan because Juan didn't continuously remind Rodrigo of his own faults, but the conversation is interrupted by Rodrigo being poisoned. The conversation finally gets finished at the end of Season 3 (after spending most of the intervening time undermining each other in various ways,) when Rodrigo blurts out to the man trying to reconcile them "I look into his eyes, I see myself! Do you expect me to love that!?"
- Frasier: The recurring gag with Frasier's Sit Com Arch Nemesis, Cam Winston, was that Cam was almost exactly like Frasier, just slightly more successful at it.
- Monica's boyfriend Pete in Season 3, who like her was a Competition Freak and Control Freak. She eventually broke up with him because she couldn't deal with his stubborness. In the next episode, her easy going best friend Chandler implies he has feelings for her, laying the groundwork for them falling in love a season later. Their subsequent relationship and marriage is a very happy one.
- To a much lesser degree, Kathy, Chandler's last girlfriend before Monica also qualifies. They initially bond over their favourite tv shows and books and share a similar sense of humour. However in hindsight they both struggle with communication in a relationship and have Commitment Issues, which leads to their break up. The stable and level-headed Monica however, helps him overcome these problems rather than making them worse.
- Non-romantic example: When Rachel starts dating Ross's doppelganger, Ross is the only person apart from Rachel who doesn't notice the similarities. And he hates Russ.
Ross: And it takes him, what? ... like ... I dunno ... uh, hello? ... a week just to get out a sentence?
Chandler: Annoying, isn't it?
- House: Dr. House gets rid of one of his candidates because they think too much alike; the purpose of House's team is to challenge him and provide avenues of investigation he wouldn't have thought of alone.
- Red Dwarf:
- The episode "Me^2", in which Rimmer's self-loathing means that two copies of him end up unable to talk to each other, with one of them openly desiring the other's death.
- In "Parallel Universe", Lister and Rimmer meet their female versions. Dave Lister thinks Deb Lister is gross for a girl, but they avert the trope as they actually get on very well. However, Arnold Rimmer is completely disgusted by Arlene Rimmer.
- On Seinfeld, Jerry (being a narcissist) falls in love with a woman, played by Janeane Garofalo, because they have so much in common; he gradually realizes (being a self-loathing narcissist) what a mistake this is. He tells Elaine how it ended: they were sitting across from each other at Monk's when they both blurted out, "I hate you," and just like that they went their separate ways, still marveling over how they were always on the same page.
- Supernatural: While Sam's main issue with his father is that he and Dean were basically raised as child soldiers, after a Not So Different moment Dean notes that this was probably a factor as well.
Dean: Y'know, I finally get why you and Dad butted heads so much. You two are practically the same person.
- On Star Trek: Voyager, the Doctor gets a chance to meet his "father," the scientist who designed him. They clash immediately. Troi, who had been trying to counsel them, eventually gets tired of their crap and exclaims that she expected them to be similar but different, "like two sides of the same coin," but now she realizes that they're exactly alike—they're both jerks!
- In Osaka Pro, Gamma was basically an expy of WCW, Toryumon System and Dragon Gate wrestler CIMA. CIMA wasn't too thrilled when Gamma jumped over and it would take years of fighting before the two finally became friends.
- Ring of Honor tag team The All Night Express were incredibly similar to Wrestling's Greatest Tag Team, even taking up similar entrance attire to the latter when they showed up in the promotion. Kenny King and Rhett Titus were quintessential jerk jocks who had spent their ROH careers belittling and bully any 'vulnerable' members of the roster but were none to pleased to be on the receiving end from Charlie Haas and Shelton Benjamin.
- Implied in Dragon Age: Inquisition. The one time Dorian mentions his betrothed, he strongly indicates that the two of them dislike each other for this reason.
- If (male) Shepard decides to break up with Miranda in Mass Effect 2, he uses this exact phrase.
Shepard: This won't work Miranda. We're too much alike.
- In The Sims 3, most traits you can give a Sim strengthen the relationship when encountered in another Sim, but the "snob" trait conflicts with itself. Snobby Sims prefer someone with the "easily impressed" or "schmoozer" trait, who will agree that the snobby sim is superior.
- In El Goonish Shive, when Tedd plays Luke at Magickal Cards, neither of them seem to notice that it's practically a Mirror Match. They do, however, both think simultaneously "This guy has a really annoying play style."
- Sluggy Freelance: In "Oceans Unmoving", this is the core of the dynamic between Bun-bun and his Arch-Enemy Captain Blacksoul. Both of them are unbeatably badass Determinators who make everyone else do exactly as they want and don't have patience for much talk. In the same story's twist ending (so you have been warned), it's also revealed that Blacksoul is also Bun-bun, from a different time, disguised inside a powerful flying robot dressed in robes. He wants to follow his earlier self to find out how he got out of Timeless Space the first time, something he can't remember. He can't just reveal himself since he knows he'd attack anyone trying that since he knows the real him would know that and not try it. He's really annoyed at his other self by the end.
- In Noob, Omega Zell is a misogynist and Gaea a woman who doesn't like being demeaned. Gaea, on the other hand, displays behaviors that Omega Zell tends to not approve of in general. Aside from this, the two don't get along simply from both being self-centered, ambitious and taking every opportunity they can to bring their goal closer. The situation is such that if one of them has a opportunity to make a big step toward their objective, they (often correctly) assume that the other has the same idea in mind.
- Alvin and Brittany in Alvin and the Chipmunks. When you have two Attention Whores with big egos headbutting, getting along isn't on the list of priorities as much as besting each other is.
- Eddy and Kevin in Ed Edd And Eddy have many similarities. They are both the leader of their respective group of friends. They have both called Jimmy "Fluffy", which nobody else has. And in the episode "Fa la la la Ed", where the kids decided to celebrate Christmas in July, they both felt that it was stupid at first, while nobody else objected to it. But they are pretty much sworn enemies, because they also have two more similarities: they both want to be the most popular kid in the Cul-de-Sac (even though Eddy has no chance of beating Kevin in that department), and they can both be huge jerks.