How to do automatic Active directory log in if the user logged in domain

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ASP.NET

Hi all

I want to do automatic active directory login when we take a page if the user logged in to the domain . I can login to the system using following code

public bool AuthenticateUser(string domain, string username, string password,string LdapPath, out string Errmsg)

        {

            Errmsg = "";

            string domainAndUsername = domain + @"\" + username;

            DirectoryEntry entry = new DirectoryEntry(LdapPath, domainAndUsername, password);

            try

            {

                // Bind to the native AdsObject to force authentication.

                Object obj = entry.NativeObject;

                DirectorySearcher search = new DirectorySearcher(entry);

                search.Filter = "(SAMAccountName=" + username + ")";

                search.PropertiesToLoad.Add("cn");

                SearchResult result = search.FindOne();

                if (null == result)

                {

                    return false;

                }

                // Update the new path to the user in the directory

                LdapPath = result.Path;

                string _filterAttribute = (String)result.Properties["cn"][0];

            }

            catch (Exception ex)

            {

                Errmsg = ex.Message;

                return false;

                throw new Exception("Error authenticating user." + ex.Message);

            }

            return true;

        }

But if the user already logged to the domain it will not be good to login again . I want user to be redirect to default.aspx if the user logged in domain . ican use the above code if i got username, password of the user logged in to the domain .Is it possible or any other methods for that?

Thankss in Advance
Amritha