Print dulicate chars in a string, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
           
            String str = "aaa b bdd h aaa k ";
            char ch = printDuplicateChars(str);
        }
        
        public static char printDuplicateChars(String str)
        {
            Dictionary<char,int> dict = new Dictionary<char,int>();
            dict.Add(str[0],1);
            for(int i=1; i< str.Length; i++)
            {
                 if(dict.ContainsKey(str[i]))
                    dict[str[i]]++;
                 else
                     dict.Add(str[i],1);
                    
            }
            foreach (KeyValuePair<char,int> keypair in dict)
            {
                if(keypair.Value>1)
                {
                    Console.Write(keypair.Key);
                }
                
            }
            return ' ';
        }
    }
}

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Compilation time: 0,14 sec, absolute running time: 0,11 sec, cpu time: 0,12 sec, average memory usage: 13 Mb, average nr of threads: 3