CareerCup

Use recursion to traverse all the files in the folder. For each file generate it's MD5 checksum and use a hash table to keep track of duplicates using MD5 as key and frecuency as value.

Use md5sum utility to compute 'md5 hashes for each fiile.
Build a hash table / map of md5hash : <list of files>
Done.
Go thru hash table, output each list as duplicates

import java.io.File;
import java.util.HashSet;
public class ReadDuplicateFileInDir {
public static void main(String[] args) {
File folder=new File("C:/Users/sumitksi/Downloads");
HashSet<File> al=new HashSet<File>();
getAllFilesRecursevely(al,folder);
for(File file:al)
{System.out.println(file);}
}
public static HashSet<File> getAllFilesRecursevely(HashSet<File> al,File folder){
for(File file:folder.listFiles())
{
if(file.isFile())
if(al.add(file));
//Do nothing.
else
System.out.println("Its duplicate :"+file);
else
getAllFilesRecursevely(al,file);
}
return al;
}
}

import java.io.File;
import java.util.HashSet;
public class ReadDuplicateFileInDir {
public static void main(String[] args) {
File folder=new File("C:/Users/sumitksi/Downloads");
HashSet<File> al=new HashSet<File>();
getAllFilesRecursevely(al,folder);
for(File file:al)
{System.out.println(file);}
}
public static HashSet<File> getAllFilesRecursevely(HashSet<File> al,File folder){
for(File file:folder.listFiles())
{
if(file.isFile())
if(al.add(file));
//Do nothing.
else
System.out.println("Its duplicate :"+file);
else
getAllFilesRecursevely(al,file);
}
return al; }}

def findDup(parentFolder):
    # Dups in format {hash:[names]}
    dups = {}
    for dirName, subdirs, fileList in os.walk(parentFolder):
        print('Scanning %s...' % dirName)
        for filename in fileList:
            # Get the path to the file
            path = os.path.join(dirName, filename)
            # Calculate hash
            file_hash = hashfile(path)
            # Add or append the file path
            if file_hash in dups:
                dups[file_hash].append(path)
            else:
                dups[file_hash] = [path]
    return dups

def findDup(parentFolder):
    # Dups in format {hash:[names]}
    dups = {}
    for dirName, subdirs, fileList in os.walk(parentFolder):
        print('Scanning %s...' % dirName)
        for filename in fileList:
            # Get the path to the file
            path = os.path.join(dirName, filename)
            # Calculate hash
            file_hash = hashfile(path)
            # Add or append the file path
            if file_hash in dups:
                dups[file_hash].append(path)
            else:
                dups[file_hash] = [path]
    return dups

def findDup(root):
    # Dups in format {hash:[names]}
    dups = {}
    for dirName, subdirs, fileList in os.walk(parentFolder):
        print('Scanning %s...' % dirName)
        for filename in fileList:
            # Get the path to the file
            path = os.path.join(dirName, filename)
            # Calculate hash
            file_hash = hashfile(path)
            # Add or append the file path
            if file_hash in dups:
                dups[file_hash].append(path)
            else:
                dups[file_hash] = [path]
    return dups

Test comment

List<String> dupFiles(String[] files) {
    List<String> res = new ArrayList<>();
    Set<String> set = new HashSet<>();
    for( String file : files) {
        if(!set.add(file)) {
            res.add(file);
        }
    }
}