Reading a txt file in C++ from a function that forces the user to enter anthore value before continuing

Question

I want the code to read through the file that is created and if they find the same string of data for student ID, or student name it will make the user enter another value before moving on. I was able to create and store the data so that it is persistent at this point but I have yet to figure out how to make the code search through the text file and find the user entered duplicates while at the same time forcing the user enter a new value.

I am unable to think of a way to accuracy show you all the code with including the entire program

 #include<iostream>
#include<fstream>
#include <cstdlib>
#include <string>
#include "student.h"
#include "course.h"
#include "session.h"
 using namespace std;


void fillStudents(student arg[], int size)
{
 for (int i = 0; i < size; i++)
 {
      cout <<"Student #"<<i+1<<endl;
      cout <<"=========="<<endl;
      cout <<"Enter a student ID (i.e 97626): ";
      cin >>arg[i].stuID;
      cout <<"Enter the first name (i.e Ryan): ";
      cin >>arg[i].fname;
      cout <<"Enter the last name (i.e Brown): ";
      cin >>arg[i].lname;
     system("CLS");
 }
 } 

  void fillCourses(course c[], int size)
 {
   for (int i = 0; i < size; i++)
   {
      cout <<"Course #"<<i+1<<endl;
      cout <<"=========="<<endl;
      cout <<"Enter course name (i.e Data-Structures (put - instead of spaces)): ";
     cin >>c[i].CourseName;
     cout <<"Enter course ID (i.e CS-230 (put - instead of spaces)): ";
     cin >>c[i].CourseID;
     cout <<"Enter number of credits (i.e 3): ";
     cin >>c[i].numberOfCredits;
     system("CLS");
 }
}

string CourseN(int choice, course ch[])
{
switch(choice)
{
    case 1:
        return ch[0].CourseID+" "+ch[0].CourseName;
        break;
    case 2:
        return ch[1].CourseID+" "+ch[1].CourseName;
        break;
    case 3:
        return ch[2].CourseID+" "+ch[2].CourseName;
        break;
    case 4:
        return ch[3].CourseID+" "+ch[3].CourseName;
        break;
    case 5:
        return ch[4].CourseID+" "+ch[4].CourseName;
        break;
    case 6:
        return ch[5].CourseID+" "+ch[5].CourseName;
        break;
    case 7:
        return ch[6].CourseID+" "+ch[6].CourseName;
        break;
    case 8:
        return ch[7].CourseID+" "+ch[7].CourseName;
        break;
    case 9:
        return ch[8].CourseID+" "+ch[8].CourseName;
        break;
    case 10:
        return ch[9].CourseID+" "+ch[9].CourseName;
        break;
    default:
        cout<<"Invalid decison!! Good Bye!"<<endl;
        exit(0);
}
}

int creditTotal(int sel, course ch[])
{
switch(sel)
{
    case 1:
        return ch[0].numberOfCredits;
        break;
    case 2:
        return ch[1].numberOfCredits;
        break;
    case 3:
        return ch[2].numberOfCredits;
        break;
    case 4:
        return ch[3].numberOfCredits;
        break;
    case 5:
        return ch[4].numberOfCredits;
        break;
    case 6:
        return ch[5].numberOfCredits;
        break;
    case 7:
        return ch[6].numberOfCredits;
        break;
    case 8:
        return ch[7].numberOfCredits;
        break;
    case 9:
        return ch[8].numberOfCredits;
        break;
    case 10:
        return ch[9].numberOfCredits;
        break;
    default:
        cout<<"Invalid Decision!! Good Bye!"<<endl;
        exit(0);
    }
}

void fillSession(student arg[], course c[], session s[])
{
int c1, c2, c3, c4;
string ch1, ch2, ch3, ch4;
string startDate, endDate;
int cr1, cr2, cr3, cr4;

   for (int i = 0; i < 4; i++)
   {
    for(int j = 0; j < 10; j++)
    {
        cout<<"class #"<<j+1<<" \t\t"<<c[j].CourseID<<" \t\t"<<c[j].CourseName<<" \t\t\t"<<"Credits: "<<c[j].numberOfCredits<<"\n\n";
    }

    cout<<"Enter your 1st class choice: ";
    cin>>c1;
    ch1=CourseN(c1,c);
    cout<<"Enter your 2nd class choice: ";
    cin>>c2;
    ch2=CourseN(c2,c);
    cout<<"Enter your 3rd class choice: ";
    cin>>c3;
    ch3=CourseN(c3,c);
    cout<<"Enter your 4th class choice: ";
    cin>>c4;
    ch4=CourseN(c4,c);

    s[i].courseID=ch1+"\n"+ch2+"\n"+ch3+"\n"+ch4;
    cout<<"Enter a start date for your classes (i.e mm/dd/yyyy): ";
    cin>>startDate;
    s[i].startDate=startDate;
    cout<<"Enter a end date for your classes (i.e mm/dd/yyyy): ";
    cin>>endDate;
    s[i].endDate=endDate;

    cr1=creditTotal(c1, c);
    cr2=creditTotal(c2, c);
    cr3=creditTotal(c3, c);
    cr4=creditTotal(c4, c);

    s[i].totalCredits=cr1+cr2+cr3+cr4;

    c1=0;
    c2=0;
    c3=0;
    c4=0;

    system("CLS");
  }
 }

void display_report(student s[], course c[], session se[])
{
  for (int i = 0; i < 4; i++)
  {
    cout<<"Student ID: "<<s[i].stuID<<"\n"<<endl;
    cout<<"Student Name: "<<s[i].lname<<", "<<s[i].fname<<"\n"<<endl;
    cout<<"Course IDs: \n"<<se[i].courseID<<"\n"<<endl;
    cout<<"Start Date: \n"<<se[i].startDate<<"\n"<<endl;
    cout<<"End Date: \n"<<se[i].endDate<<"\n"<<endl;
    cout<<"Total Credits: "<<se[i].totalCredits<<"\n"<<endl;
    cout<<"\n"<<endl;
   }
}

This is the function that I am using to create the file and continuously store the user entered information. it is within this function that I would want to try to search for the duplicate words

 /**
 * Writes session data to file.
 * @param   {session}   sess   sessions array.
 */
void writeSessionsToFile(session* sess, student* stude)
 {
  ofstream file_out("sessions.txt", ios::app);
  // for now let us just assume there'll always be just 4 sessions.
   for (size_t i = 0; i < 4; ++i)
  {
   file_out << stude[i].lname << ", "<<stude[i].fname<<"\n";
   file_out << stude[i].stuID << "\n";
   file_out << sess[i].courseID << "\n";
   file_out << sess[i].startDate << "\n";
   file_out << sess[i].endDate << "\n";
   file_out << sess[i].totalCredits << "\n";
   // empty line in the end of each entry.
   file_out << endl;
 }
}


int main()
{
cout<<"Welcome to the CS 230 Data Structures class add program!"<<endl;
cout<<"Please follow the prompt to gain a report of your courses for the year: "<<endl<<endl;
//declare and initialize 4 students
student stu[4];
fillStudents(stu, 4);

//declare and initialize 10 courses
course courses[10];
fillCourses(courses, 10);

//assign 4 courses for each student
cout<<"Enter a class selection based on the chart above (1-10), please do not choose the same class twice: "<<endl;
cout<<"Each student goes by how their information was added"<<endl<<endl;

session sessions[4];
fillSession(stu, courses, sessions);

writeSessionsToFile(sessions, stu);

//display a report showing each student with his/her courses
//show the total number of credits for each student
cout<<"Please find your name and other important information below to see your classes and credit totals: \n\n"<<endl;
cout <<"=========="<<endl;
display_report(stu, courses, sessions);
cout <<"=========="<<endl;


// instead of system(pause), pause doesn't work on linux.
cin.get();
return 0;
}

I have searched the internet as well as this website I have found function that counts the lines of the word that I am searching for, as well as functions that does find and forces the user to enter another value but I have yet to find one that I could implement in my code that will allow me to continue with the goal I have in mind