See this topic: http://gathering.tweakers.net/forum/list_messages/1568620 (Google translated version: http://riii.nl/qm7te )

duplicatefinder.cs

using System; 
using System.Collections.Generic; 
using System.IO; 
using System.Linq; 

class Program 
{ 
    static void Main(string[] args) 
    { 
        var duplicates = new DirectoryInfo(@"G:\sampling") 
            .GetFiles("*.*", SearchOption.AllDirectories) 
            .GroupBy(f => f, new FileComparer())    //Group by file equality 
            .Where(g => g.Count() > 1);             //Groups with more than 1 file 
        //Voila! 
    } 
} 

public class FileComparer : IEqualityComparer<FileInfo> 
{ 
    public bool Equals(FileInfo left, FileInfo right) 
    { 
        if (left.Length != right.Length) 
            return false; 

        Int64 l = 0, r = 0;         //"Buffers"  
        long p = 0,                 //Position  
             i = left.Length / 8,   //Iterations of "int64-sized reads"  
             c = left.Length % 8;   //Rest  

        using (var brl = new BinaryReader(left.OpenRead())) 
        using (var brr = new BinaryReader(right.OpenRead())) 
        { 
            //Compare "int64 chunks" at a time  
            while (l == r && p++ < i) 
            { 
                l = brl.ReadInt64(); 
                r = brr.ReadInt64(); 
            } 

            //Compare remaining bytes (if any and the left still equalled right in the last read)  
            if (l == r && c > 0) 
            { 
                p = 0;  //Reset position  
                while (l == r && p++ < c) 
                { 
                    l = brl.ReadByte(); 
                    r = brr.ReadByte(); 
                } 
            } 
        } 
        return l == r; 
    } 

    public int GetHashCode(FileInfo obj) 
    { 
        return unchecked((int)obj.Length);  //As a first "indicator" the filelength is used as "hashcode" 
    } 
}