Well I think I failed that test

With forensics being as good as they are now, they'll probably figure out the knife you used the first time is the same knife you used the second time.

You really can't have a favourite knife anymore. Gotta dispose of it right away or they'll work out the pattern without needing to figure out the riddles you left behind at the scene. It's cheating really, but the only thing you can do to prevent it is by taking the "single-use knife" approach.

It's the kind of knife you buy for your first apartment