Remove duplicates from Arraylist populated by Active Directory

Over here, I've got a problem with out Exchange 2003 Server, probably during migration from 5.5 -> 2003 some mail addresses were duplicated with different CNs.
The following code should sort a list of email adresses, which I recieve from our AD (contacts.Add(contact.Properties["mail"].Value.ToString());) and print out duplicated ones.

#####
contacts.Sort();
for (int i=1; i <= contacts.Count-1; i++)
{
      Console.WriteLine(contacts[i]);
      Console.WriteLine(contacts[i-1]);
      if(contacts[i] == contacts[i-1])
      {
            Console.WriteLine("Duplicate: "+contacts[i]);
      }
}
#####

As you can see, I'm outputting the email addresses it's going to check, and yes, they are the same - of course. I'm searching for the exact email adresse with objSearcher.Filter = "(mail=frank.lattuca@saarland.ikk.de)";

However the if condition returns false.
Now, if I'd check if the .GetHashCode() of contacts[x] are the same, it returns true.

Am I save to use the hash for comparing?
Why aren't the equal strings equal?

FYI: contacts is a System.Collections.ArrayList