This was brought up at a course I attended recently and I think they said there's a sort of dummy-submission process where you can pre-check something before the uni do a full submission and check, so that you don't end up plaigiarising yourself.

We were also warned that it's not unusual for non-plaigiarised material to get quite a lot of matches now as there is so much material on there and only so many ways of saying certain things. So this shouldn't just be judged with algorithms - and don't freak out if there is some degree of matching when you haven't done anything wrong! Just check with someone that the level of matching is ok.