People seem to like these plotlines... here's how to make them better.

Harry Potter's Twin

While it is somewhat reasonable to give Harry a twin, and even plausible that said twin might get struck by the Killing Curse instead of Harry, there are some limits. 

You cannot make the single curse strike both of them without drastically changing the events of that Halloween night. To preserve the same basic chain of events, the curse would have to hit one twin, bounce, hit the other, bounce, and hit Voldemort. There are multiple things wrong with this idea. Firstly, the necessary reflection angles would be entirely unlikely to happen. Secondly, even if the curse did somehow bounce at the exact necessary angles, Voldemort would have more time to process the fact that his curse bounced and dodge it than he did with the curse only bouncing off one child. 

However, if you want to have two people casting Killing Curses, maybe Voldemort and Bellatrix, and they both get their curses reflected back at them, go ahead. Just make sure that nobody magically decides "this twin is the one who survived the Dark Lord's curse" when both twins have curse scars and there should be no way to tell which curse was cast by who. Also, make sure to consider the following paragraph. 

In cannon, Lily sacrificed herself to save her child. However, with two children instead of one, a single sacrifice would not necessarily be enough. It might, but that doesn't mean you should risk it. Both of Harry's parents die anyways, so you might as well have James sacrifice himself along with Lily, instead of rushing to confront Voldemort and just plain dying. 

And finally, if you don't go for the both-were-struck route, don't automatically make Harry's twin the Boy (or Girl) Who Lived. If you can create an interesting plotline revolving around the child of prophecy having a twin, it shouldn't really matter which twin fills which role for the sake of the plot, so it's probably best to minimize the initial changes. Remember, this doesn't mean you can't change some things around later down the line, though it's preferable for them to be spreading ripples of your initial changes, not entirely new changes. Harry could easily wind up with a very different personality growing up with a twin than by himself, especially if Dumbledore decides two extra kids is too much to ask of the Dursleys and doesn't send either of them there. Keep in mind, if you go with that idea, the fact that having a twin living in the same house as Harry could be a basis for blood wards just as much as living with the Dursleys.