I think I should give you a warning and say, right away, that this post might come off as ranty. This is a topic on which I am easily annoyed.
Twins are cool.* Readers and writers seem to like the idea of two human being so close they spent nine months stuck next to one another in their mom's womb. Like it makes them share something extra special.
Here's the problem. There is a ton of clichés about twins in fantasy, and threading too close to almost every one of them will make your twins (or their relationship) two-dimensional and mediocre. I'm extra-sensitive to this, probably because I have a twin sister and some of these spill over in real life.
So, let's take a look at them!
The Telepathic Twins
This is the one I hear the most about outside of novels and movies. I don't know why and when people started to believe twins had this telepathic link between them. They don't. I don't get sad when my sister is sad. I don't feel her pains and joys. And I certainly don't hear her voice in my head, though I sure can imagine what she'd say. This is an annoying myth in real-life.
In speculative fiction, well, you have a little more leeway. Magic could explain the telepathy. In a world where such connections are frequent between close friends or members of the same family, well, I'm willing to admit twins might have increased chance of developing a link.
If the only trace of telepathy in your novels, ever, is those two shiny special twins... Cut it. Seriously. Randomly giving twins powers no one else can have is bad. It's a serious affront to all your worldbuilding. It discredits the characters. It discredits the story. Just don't do it unless it is completely in line with everything else.
And even then, consider not doing it. For me, m'kay?
The Mirror Twins
This is perhaps the most frequent and most annoying of them, because it reeks of rushed characterization. There's typically two types of mirror: the morality mirror and the ability mirror.
The first is our classic Good Twin/Evil Twin. Two twins were separated at birth, one grows to be a valorous knight and the other an evil conqueror? I'm sure that rings a bell. Or perhaps they grew together, and one kept bullying the other.
The problem here is that there is often little to no explanations as to why one turns out so well and not the other. You get a little more room when they were separated, but the moment your readers see "twins separated at birth", they're likely to roll your eyes and not give you the chance.
The morality mirror twins is alluring, I know. It's a blast when done well. There are ways to make it less cliché. Play in the shades of gray instead of having it all black and white. Put the Good Twin on the wrong side, maybe? If you must make one change the others' belief, why not make it the evil twin convincing his gentle counterpart? There are other options here. If you must have a good and an evil twin, play with them.
The second, the ability mirror, can actually be combined with the first for maximum Claudie-Facepalm. The ability is when one twins' powers or appearance is the exact opposite of his other twins. One has Fire Magic, the other Water. One is a great warrior, the other a scrawny wizard. You know the drill. Intelligent vs Stupid. Paranoid vs Naive. Blond hair vs Black hair. Scientist vs Artist. The physical ability mirror is very frequent with fraternal twins.
Because really, if they're not going to look the same, they must look opposite, otherwise what's the point? (Feel free to imagine me banging my head on the desk. Because I am)
The Copycat Twins (and the switch-place plot)
We all have read a novel where one twin takes the other's place. Where identical twins are concerned, this is a frequent plot device. It can work, if they aren't supposed to switch for a long time.
This is one of those I'm less bothered with. Most of the time when a writer does this, he has taken the time to give distinct personality to the twins, and the difficulty of hiding the differences is where the tension comes from.
Here's a reminder, though: identical twins, despite their names, aren't the exact same. There are always differences that allow a quick identification, especially as the twins get older (because environmental factors have an increasingly big influence on appearance). Yes, twins can switch place. In face of those who know them well, however, it isn't likely to last.
The Point of This
Here's the message, because I'm not just writing this to vent. I swear I'm not!
Twins, identical or not, are two separate human beings. They deserve to have complex personalities, unique outlooks on life and abilities that do not depend on their other twin. They have to be someone on their own. Build your twins as two humans who just happened to be born on the same day, not as a single unit meant to be together.
To end on a positive note, there are a lot of cool twins in litterature. I don't hate twins by default. In fact, when you get past my initial reserves, I tend to give unconditional love to cool twin characters. Here are a few...
Jaime and Cersei Lannister, from A Song of Ice and Fire (though I do hate Cersei on her own. grr)
Fred and George Weasley, from Harry Potter. (of course they're here!)
Raistlin and Cameron Majere, from the Dragonlance Chronicles. (they're a case study of all these cliches done in a way that actually works for me!)
There are others, such as Luke and Leila. And I bet you can come up with more! Who are your favourite twins?