ShapeShift holds relatively small hot wallets in order to process instant transactions, so there is a pretty decent chance that you will end up getting the exact same coins back. However, there is no guarantee as it is random.
It's important to note that ShapeShift is not designed nor intended to be used as a mixing or tumbling service. We do not endorse the use of ShapeShift in such a manner and it likely will not achieve your goals if you try to use it in such a way. There is a good chance that you will get the same coins back that you started with.
Please note that any clearly suspicious activity may be reported to relevant authorities.