Having the police, prosecution and lab working together as is apparent in this case is asking for problems. It doesn't need to rise to the level of a conspiracy but simple confirmation bias when one "knows" what the results are supposed to be.
Timely collection and securing of the evidence goes a long way to eliminating the window for planting evidence. If the evidence is bagged before anyone has an opportunity to assess who could be a suspect, then there is a high probability that planted evidence will conflict with the fingered suspects alibi which would point a very damning finger back at whoever had the opportunity to do such planting. In this case however, we have a knife unpacked and repackaged in the police station after the suspects had been interrogated and arrested and the bra clasp that was discovered the day after the crime was supposedly not collected until 47 days later after all the other evidence failed to link Raffaele to the scene.
If the bra and knife DNA evidence are upheld and the possibility of contamination is rejected and the computer evidence proves Raffaele could not have left his apartment that night, what is left to explain the existence of the evidence?