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there is already an open datareader associated with this command which must be closed first.+vb.net

there is already an open datareader associated with this command which must be closed first.+vb.net
I am closing reader.Close() for each datareaer.
code is working in a single mechine properly.
more request comes from two or more mechine then i get an Error
Error-->:there is already an open datareader associated with this command which must be closed first.

Protected Sub BtnLogin_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles BtnLogin.Click
 
            Dim sss As String = cn.State.ToString()
            Session("UId") = TBUserName.Text
            Dim ID As String = TBUserName.Text
            cmd = New SqlCommand
            cmd.Connection = cn
            cmd.CommandType = CommandType.StoredProcedure
            cmd.CommandText = "sp_administration"
            cmd.Parameters.Add("@transid", SqlDbType.Int).Value = 1
            cmd.Parameters.Add("@userid", SqlDbType.VarChar, 50).Value = TBUserName.Text
            cmd.Parameters.Add("@password", SqlDbType.VarChar, 50).Value = TBPassword.Text
 
            rd = cmd.ExecuteReader()  ///Error
          
 
            If rd.Read() Then
                Session("AId") = rd.GetValue(0).ToString()
                Session("CenterId") = rd.GetValue(3)
                Session("CID") = rd.GetValue(3)
                Dim userType As String = rd.GetValue(2).ToString()
                rd.Close()
                Session("UId") = TBUserName.Text
                Session("Utype") = userType.ToString()
                If userType = "0" Then
 
                    Response.Redirect("frmExamDescription.aspx")
                End If
                If userType = "1" Then
 
                    Response.Redirect("frmAdmin.aspx")
                End If
 
                If userType = "2" Then
 
                    Response.Redirect("GeneralExamShedule.aspx")
                End If
            Else
                lblmsg.Text = "Invalid Userid/password"
            End If
            rd.Close()
          
        End Sub
 
 
Connection is openened in a session start event

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