The results of checking against existing and other test takers solutions must be taken with a strong human judgment. Programming problems such as would be asked in tests are essentially like mathematical formulas/algorithms, and there isn't much variation in how a given formula or algorithm can be implemented.
I don't think these techniques are often applied to problems in tests - there are other, simpler ways of catching cheaters there.
They are much more likely to be applied to homework assignments, where the opportunity for copying is large, but the chance of two students producing the exact same >500-1000 line program is slim to none. Perhaps once in a while a critical function will be copied and no one will realize or similarities in a trivial function will be unnecessarily flagged, but this will be relatively rare and quickly discovered in manual review.
There is a lot of syntactic variation possible, both for formula and algorithms. Even for something as simple as quicksort there is enough natural variation for a class of 30 maybe even 100 (if no references can be used).
Anything more complex and even with references it should be unique.
