I say everything twice?

You might want to research echolalia. I am not confident in my knowledge about echolalia, but it sounds like it could be what you are experiencing. It is often a symptom of different mental health problems (like autism) and I think it can be exacerbated by stress.

You stutter???

Jimmy Two Times, a character in Goodfellas, does exactly the same thing. I think it's pretty nifty. Pretty nifty.

What?What?

But you didn't type everything twice here.

But you didn't type everything twice here.

It's most likely something neurologically related. Go see a neurologist.

I used to do this when I was a kid. I'd say the sentence and then, so quietly I was pretty much just mouthing it, would say it again under my breath. It comes up again if I'm stressed or distressed.
Don't worry about it. Everyone has quirks. It's probably the sign of a deeper problem, but (having studied psychology) I can assure you that everyone has every single mental disorder, just to lesser and greater degrees.

I do it on occasion but it doesn't bother me cause I know I have Aspergers syndrome.

See a doctor

It's definitely not a stutter roxco.

I think you should go and see a doctor if it's bothering you but it's not to worry about, many people have this problem and it comes in several different forms

You're normal. You're normal.

OCD

My friend does it three times and she doesn't even notice, so me and my other friend make jokes to her face, like "I'M GOING TO KILL YOU...I'm going to kill you... i'm going to kill you"

Funny thing is she never noticed until we start poking fun at her, thinking she was completely aware of her creepy behavior. She laughs with us but I kind of wish she got help. I know she is very anti-social and suffering from epilepsy/half brain damage (from being in a coma) and probably suffering from aspergers syndrome which would explain so much.

i used to do this all the time! i kinda stopped now but i still do it sometimes

I hate to throw out different mental disorders as reasons for your ailment, but I really do think you should see a doctor about it. Repeating things can be a sign of OCD, Autism, and even simple Social Anxiety. Echolalia, as previously mentioned by BfingIToucher, is when you parrot other peoples' words. Some people repeat things involuntarily or to make things more socially acceptable (in their mind) or as a sign of acceptance of their thoughts or statements. I really think you need to talk to your doctor about it.