ALL lab results that verify a crime should be confirmed by A SEPARATE LAB. Additionally, no samples should contain a name or details of the alleged crime.
IF both tests are positive with no chance for impropriety or prejudicial outcomes, then justice should be swift and severe.
Chemical analysis usually involves destroying part of the sample, and forensic sample sizes are often small to begin with. There is also the matter of doubling the cost.
Another way to prevent this is just to randomly send the lab control samples of substances where you already know the composition, and see what results the lab sends back. When they tell you that the control sample of powdered sugar you sent is cocaine, then you know you have a problem on your hands.