Italian law prohibits a version of double jeopardy — being tried anew for a crime for which you have already been cleared, said Praxilla Trabattoni, an Italian lawyer who was followed the case. This case is technically different.
Trabattoni said that the Supreme Court was essentially saying that "when the appeals court was evaluating whether she did it or didn’t, the appeals court did that on the basis of evidence that shouldn’t have been admitted.”
Italian law says that a judgment is not definitive until it’s cleared every degree of trial, Trabattoni said, and the Supreme Court is considered the third degree of trial, after the lower court and the appeals court. If the Supreme Court had upheld the acquittal and then prosecutors had brought a new case entirely, that would be considered double jeopardy under the Italian system, Trabattoni said.
If re-convicted in absentia, Italy likely won't seek extradition. Due to the rampant corruption and the highly political nature of their justice system, Italy probably wants to make sure that Knox never steps foot on Italian soil again because that would start a whole new round of tabloid and news reporting, and the whole case is an embarrassment.