"Didn't" is *meant* to be two syllables: "DID-unt". That's how it sounds virtually every time i hear it (in Seattle, US). But, yeah, like Alaina says some folks shorten it to one syllable, either due to accent or some other affectation. For example, I grew up in the north of England and it was pronounced as the single syllable "dint" by most people, most of the time.

When saying didn't (the short version for did not) we often say it really fast, leaving out the second "d" and the "t". Thus, it may sound more like, "She din do it" instead of "She didn't do it".