display repeated lines in a file

import java.io.*;
import java.util.*;
class  ReadDuplicateLinesFromFile
{
    public static <T> List getDuplicate(Collection<T> list) {

    final List<T> l = new ArrayList<T>();
    Set<T> set = new HashSet<T>() {
    public boolean add(T e) {
        if (contains(e)) {
            l.add(e);
        }
        return super.add(e);
    }
   };
   for (T t : list){
        set.add(t);
    }
    return l;
}

public static <T> boolean hasDuplicate(Collection<T> list) {
    if(getDuplicate(list).isEmpty())
        return false;
    return true;
}
    public static void main(String[] args) throws Exception
    {
        Scanner input=new Scanner(System.in);
        System.out.print("Enter file name: ");
        String filename=input.next();
        File f=new File(filename);
        BufferedReader br=new BufferedReader(new FileReader(f));
        ArrayList list=new ArrayList();
        String st="";
        while((st=br.readLine())!=null){
            list.add(st);
        }
        System.out.println("Duplicate Elements are: ");
        if(hasDuplicate(list)){
            List l=getDuplicate(list);
            for(int i=0;i<l.size();i++){
                System.out.println(l.get(i).toString());
            }
        }
    }
}