[DotNetDevelopment] How to maintain mulptiple session for same session variable

[kp]

here is my issue:

I have my coding like this

For Login page:

    int cnt = 0;
    /*Connection to the database*/

   SqlCommand cmd = new SqlCommand("select
e.e_code,e.password,c.cat_type from Employee_master e, Category_master
c where c.cat_id=e.cat_id and e.e_code='" + txt_user.Text + "' and
e.password='" + txt_password.Text + "'", cn);

   SqlDataAdapter adpt = new SqlDataAdapter(cmd);
   /*Collected the count in data table*/
            cnt = dt.Rows.Count;

            if (cnt > 0)
            {
                type = Convert.ToString(dt.Rows[0][2]);
                Session["e_code"] = txt_user.Text;
                Session["Role"] = type; //this is where i assign my
role of user who logged in

                if (str != null)
                {
                    Response.Redirect(str + "?user=" + type);
                }
                else
                {
                    Response.Redirect("~/Admin/chome.aspx?user=" +
type);
                }
            }

For Admin Master Page:
       Static string user; //declared globally

       if (user == null || user=="")
        {
            user = Request.QueryString["user"];
        }
        else
        {
            //string str = Session["e_code"].ToString();
            user = Session["Role"].ToString();
        }

The issue is that when user1 enter the login page he is allowed to
login but when a second user enters on another client at that time the
user variable is cheked and at that time the value that is obtained is
of user1 role.

so in admin master page the if condition is false and pointer is moved
to else where the session values is null and getting object reference
not set to an instance.