When it comes to the question of putting together/discovering/unveiling a character, or however else you care to put it, there comes a time when you need to determine whether or not the character in question is a unique individual.
Most human beings are, in my estimation, unique, at least in some respect. Heck, my sister and I share parents, a best friend, schools for K-12, and DNA, and we somehow pulled off being different people. Thus, I feel reasonably sure that everyone else on the planet can certainly manage to be an individual in some respect.
That said, some people have a lot in common with other people. Pretty much everyone I know has read and loved Harry Potter. Some more devotedly than others, but they've all read it. Most girls I know read Twilight. They didn't all like it, but they've probably read it. Most people like sweets, summer vacation, and funny movies. Some things are universal.
That doesn't, though, give us as writers the excuse to default to stereotypes. When we do that, we both perpetuate often ridiculous conceptions of certain groups and create characters that no one's going to believe. (I don't know about the rest of the world, but in my experience, a person who actually aligns with the stereotype is more the exception to the rule than the rule itself.)
And even the most original person can become a fiction stereotype. I've actually created a genre in my head of books in which my friend-- let's call her Lee -- dies in the middle. Lee is a dear girl and definitely not traditional. She's the sort of girl who likes tattoos, dies her hair, wants to study Gaelic, is intensely loyal to her friends, and always seems to know a person in need to help. She flips off authority but has an ethos she believes in. I'm sure you've seen her in a book, even if you've never met her in person. Somehow, one of the most exceptional people I've ever met has become something everyone's seen in a book or movie.
(I kid you not, I once lent her a book, because I thought that, as she bore so strong a resemblance to the supporting character, she'd like it.
Me: What'd you think?
Lee: I hated it.
Lee: I DIE IN THE MIDDLE!
Me: Oh, right. That.
Probably should have realized she wouldn't be keen on that bit. She's since grown fond of the book and once gave someone a copy as a means of understanding her.)
When I see that same character over and over again, it makes me wonder if the writers know how many times someone's written her. They change the back story and the reasons why, but they end up with the same girl anyway, and she looks mighty familiar.
When I create a character, I try to make sure he or she isn't someone I know. Then I make sure he or she isn't someone I've read or seen. If my character is someone else or someone else's work, I'm not doing my job right. I don't know my character well enough, or I'm not painting them clear enough. I'm letting my characters become cliche.
Do you ever see characters in books, movies, radio shows, etc. that you feel like you've seen before? Did it bother you? Do you ever feel like someone you know appeared in a book you read? You wrote? Have you ever seen a character that bore a startling resemblance to you?