Rule 3: Twins
You cannot imagine how many fanfics I've read and they go something like this:

Hello, my name is Flora Potter, even though I'm in no way related to Harry Potter even though we have the same surname and we look exactly alike. And even though I live in the wizard in community I have no idea who he is.
      So, I'm in my cabin on the Hogwarts train when a guys walks past. He looks exactly like me but he has a lightning scar on his forehead. I run up to him and scream, "HEY YOU HAVE MY FACE, WHY DO YOU HAVE MY FACE?" and he says "Oi mate I'm Harry frickin PotTER OMG YOU MUST BE MY LONG LOST TWIN SISTER"
And then I marry Draco and stuff.

Why does everybody have a missing twin nowadays? Like, I want a twin. Twins are cute. They can wear matching clothes and stuff and they can play pranks on people and make cute Pinterest posts when they're babies............I'm going off track, sorry. Anyhow, the only person with a missing twin here is George.
A/N I know I'm heartless

What is wrong with fanfic authors nowadays, plots are all the same. Or maybe we're just expecting too much. Meh.