What's the proper interpretation of: "I was kissed by a girl twice"?

The other day I experienced two distinct but similar events. I wanted to convey this to a friend and this is what came to mind: "I was kissed by a girl twice." My question is, does this sentence say, "I was kissed by the same girl twice," or "I was kissed by two different girls on two separate occasions?"

Similarly, what's the proper interpretation of this sentence: "He got hit by a bicycle twice yesterday." (Was he hit by the same bike twice?)

I don't agree with @Alenanno's answer. The sentences you provided are ambiguous and it simply isn't clear at all whether it was the same girl/bike or not.

When I read the sentence "I was kissed by a girl twice", I first though it were 2 separate girls on 2 separate occasions, then I read @Alenanno's answer and thought, "what the ... ?"

Also, in my mind, "He got hit by a bicycle twice yesterday" means he got hit by two different bicycles for the fact of his being an asshole. I don't see how 1 cyclist could cause 2 collisions with 1 person, sort of.

Short answer: The sentences are ambiguous, there's no definitive explanation of what they mean, only how people will perceive them.

How to deal with scope ambiguity is a big topic in computational linguistics. I could write a long answer, but it's been dealt with before on stackoverflow.

To answer the main question: there is no clear-cut, proper interpretation. Without context, it is declared ambiguous.

I was kissed twice doesn't say anything about how many girls; I was kissed by two girls says nothing about the number of kisses. I was kissed by a girl twice is ambiguous (probably one girl, possibly two). He got hit by a bicycle twice yesterday is similarly ambiguous, but more likely refers to two separate bicycles, simply because you're more likely to kiss the same girl twice than you are to be hit by the same bike twice.

A girl can refer to any girl, or one girl in particular, and you need to explicitly say which.

Perhaps this is as clear as you can get:

Yesterday I was kissed once each by two [separate] girls.

It is the passive that gets me.

I was kissed twice by a girl—perhaps I was in a room full of kissing people, boys and girls and one girl kissed me twice or two girls kissed me once.

If you turn it around

Yesterday a girl kissed me twice–then I have no doubt it was the same girl doing the kissing