This task is easiest done with a boolean array. Each element is initially set to false (indicating that specific question has not yet been answered correctly). When a question is answered correctly, that entry in the boolean array is set to true.
To pick a question, use a while loop (that is primed with a question number picked randomly) that continues to pick new random numbers until that entry in the array is false (i.e. not answered correctly). Eventually it will find one