So sorry you had to file an amended return. I've had to do that a time or two. Not fun, but TurboTax does make it pretty easy!
Did you manually enter the dividend twice? Or did you import this information from a financial institution?
Either way, TurboTax wouldn't automatically catch a duplicate entry. The program doesn't know if the information entered is correct, but instead just works off of what has been entered.
TurboTax is (unless you upgrade) a self-prepared tax preparation software. What you enter or import is what stays.
One other question: Did you use TurboTax LIVE or have a TT Expert review your return prior to filing? If that's the case, please post back so that can be addressed.