There are 3 Precogs in the film, but in reality the female one is the most powerful and the twins seem to only function as amplifiers. It's revealed that there is sometimes/often a disagreement between their visions and that they do a 2/3 selection or something like that and discard the disagreeing vision (the 'minority report'). I believe this is what happened to the main character when he is framed, the stronger female's dissenting vision is ignored in favor of the two amplifiers. In the film it is ambiguous if he even commits the murder at all, the guy he kills was paid to die and more or less forces him to kill him in the struggle.

There's also the echos of visions that occur which is how the villain manages to get away with a murder, he uses his inside knowledge to copy cat another murder method which allows him to have it written off as an echo.

The wilder question in the movie is how the precogs are randomly created mutations in response to the mother taking a weird street drug during pregnancy. They'd have to dose pregnant women and hope they gave birth to more mutants if they ever want to replace the set they have, and I believe they're planning to roll it out nationwide. But given how crappy the precogs' lives seem to be maybe that is just a cost they consider worth paying.