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        // Initalize addID variable to zero to only accept integer values
        int addID = 0;

        // Validation for Student ID
        if(in.hasNextInt()){
            addID = in.nextInt();
            for (int i = 0; i < studentList.size(); i++) {
                if (addID == studentList.get(i).getID()) {
                    System.out.println("ID is already registered!");
                    Main.sysPause();
                    return;
                }
            }
        }
        else {
            System.out.println("Please enter a valid input!");
            Main.sysPause();
            return;
        }