After resetting user's password in Active Directory using C#, both old and new passwords are working

Question

Below is the code which is being used to reset the password.
I want to stop this behavior.
Only new password should work. user should not be able to log in with old password.

             using (var search= new DirectorySearcher(dir))
                {
                    search.Asynchronous = false;
                    search.CacheResults = false;

                    dirSearch.Filter = "(&(objectCategory=User)(objectClass=person)(name=" + UserName.Trim() + "))";

                    SearchResult result = dirSearch.FindOne();

                    if (result != null)
                    {
                        using (var entryUpdate = result.GetDirectoryEntry())
                        {

                            entryUpdate.Invoke("setpassword", new object[] { NewPassword });
                            entryUpdate.CommitChanges();

                            //entryUpdate.RefreshCache();

                        }
                    }
                    result = null;
                }

Answer 1

It's only possible to have two different passwords at the same time when Active Directory replication is broken. This is not actually a code issue. The way to fix it is to determine where the AD replication is broken. You can quickly check AD Health at a glance by running the command repadmin /showrepl. If you see errors, then run dcdiag /v on any domain controllers showing errors in the output. A new favorite tool of mine now to determine AD Health also is to run the PowerShell utility ADHealthCheck.