The only evidence collected from the cottage containing a sample of his DNA (mixed with Knox's)was found on a cigarette butt in the kitchen (i may be mistaken but from what I can see this was taken into evidence sometime 2-4th) The bra clasp was found 43 days after the murder happened, in a sealed crime scene.
I don't think it could happen how you said above, because they were collected at different times. Although I am still trying to learn about this so correct me if I am wrong.
What are the ways that the DNA can be transferred from the cigarette butt, that was collected over 40 days earlier onto the bra clasp?
You mention dirty gloves, so does this mean that someone would have to have handled the cig, took the gloves off, saved them and put them back on to handle the bra clasp.
Is another possiblity is someone could have touched the bra clasp on the day the cig was found, then pretended that it wasn't there and none of the technicians had seen it?
Also at this point, I am assuming they had no idea that the DNA on the cig belonged to him, so the above theory makes no sense. What are the other possibilities?
Also the DNA was mixed with Amandas so would hers not show up to?
http://themurderofmeredithkercher.com/The_Bra_Clasp#Tertiary_Transfer