How would i prevent a user from duplicate entries (Using JOptionPane and if statements in this program)

So I'm currently writing a program that basically asks for user input and calculates total course load and fees based on that output.

I'm a bit stuck, however. I'm supposed to prevent the user from entering the same course number more than once and I can't seem to figure it out. Any help would be appreciated!

This is a small piece of my code:

        while (option == JOptionPane.YES_OPTION) {
        if (!(typeOfStudent.equals("Online") || typeOfStudent.equals("On campus"))) {
            JOptionPane.showMessageDialog(null, "Enter either 'Online' or 'On campus'");
            System.exit(0);
        }
            courseNumber = JOptionPane.showInputDialog("Enter the Course Number for the class you are taking(100/150/250/300): " + "\n");
            courseNum = Integer.parseInt(courseNumber);

       if (!(courseNum == 100 || courseNum == 150 || courseNum == 250 || courseNum == 300)) {
            JOptionPane.showMessageDialog(null, "Enter a valid course number (100/150/250/300)");
            System.exit(0);
        }
        if (courseNum == 100 && (typeOfStudent.equals("Online"))) {
            totalNumCredits += credits100;
            totalStudentFee += onlineStudentFee100;

        }

Solution

Maintain a list of courses that are already selected by the user:

List<Integer> courses = new ArrayList<>();

Then modify your current code to check if the course is already selected or not.

if (!(courseNum == 100 || courseNum == 150 || courseNum == 250 || courseNum == 300)) {
            JOptionPane.showMessageDialog(null, "Enter a valid course number (100/150/250/300)");
            System.exit(0);
        }
        if(courses.contains(courseNum) {
            JOptionPane.showMessageDialog(null, "It is an already selected course number");
            return;
        }
        courses.add(courseNum);
        if (courseNum == 100 && (typeOfStudent.equals("Online"))) {
            totalNumCredits += credits100;
            totalStudentFee += onlineStudentFee100;

        }