How much seen material/questions should an exam paper contain? At University,say the lecturers hand out past papers with solutions for the students to revise from for an upcoming exam. 

Is it common practise for the exam to contain a couple of the exact same questions 
which are in the previous past exams? 

Is it common for the exam to contain questions which the lecturer has put in the assessed worksheets throughout the semester but has already handed out the solutions to?
If you hadn't included the "graduate school" tag, I would say this is both common and appropriate. My exams for undergrads are about half familiar material, that I've specifically indicated to them could appear on the exam. That way, most everyone who studies can at least pass.

But ideally, grad school should be more challenging than that.