A Relation named 'Patient' already belongs to this DataSet. How do you get a duplicate relation when only one relation is configured?

I am trying to setup a relation between two tables but keep getting this error message trapped.

A Relation named 'Patient' already belongs to this DataSet.

Can someone point me in the right direction?

Here is my code:
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protected void btnPreAuthSearch_Click(object sender, Infragistics.WebUI.WebDataInput.ButtonEventArgs e)
    {
        bool authenticated;
            authenticated = Convert.ToBoolean(Session["authenticated"]);
            
            
            if (authenticated)
            {
                  PopulatePatientDS();
                  this.PreAuthWebGrid1.DataSource = HDSDataSet.Tables["Patient"];
                  this.PreAuthWebGrid1.DisplayLayout.AutoGenerateColumns = false;
                  PreAuthWebGrid1.DisplayLayout.Bands.Clear();
                  this.PreAuthWebGrid1.DataBind();
            }

      }

      //------------   Patient Search Tab
      public DataSet HDSDataSet
      {
            get
            {
                  if (Cache["HDSDataSet"] == null)
                        Cache["HDSDataSet"] = new DataSet("HDSDataSet");

                  return (DataSet)Cache["HDSDataSet"];
            }
      }
      //------------   Patient Search Tab
      protected void PopulatePatientDS()
      {
            SqlDataAdapter sqlDA = CreatePatientAdapter();
        SqlDataAdapter sqlDAPreAuth = CreatePreAuthAdapter();
            HDSDataSet.Clear();
            sqlDA.FillSchema(HDSDataSet, SchemaType.Source);
            sqlDA.Fill(HDSDataSet, "Patient");
        sqlDAPreAuth.Fill(HDSDataSet, "Pre_Auth");

        try
        {
            HDSDataSet.Relations.Add("Patient",---------------------------------------------------DATA RELATION SET HERE   ---------------------------------
                HDSDataSet.Tables["Patient"].Columns["MPI"],
                HDSDataSet.Tables["Pre_Auth"].Columns["MPI"]);
        }
        catch (System.Exception x)
        {
            string s = x.Message;-----------------------------------------------------------------------ERROR is TRAPPED HERE  ----------------------------
            string m = s;
        }

       
    }



      //------------   Patient Adapter
      protected SqlDataAdapter CreatePatientAdapter()
      {

        string physicianNPI;
        physicianNPI = Session["physicianNPI"].ToString();

            SqlConnection conn = new SqlConnection(ConnString);
            SqlCommand cmd = new SqlCommand(SearchString, conn);
        cmd.Parameters.Add("@REFERRING_PROVIDER_NPI", SqlDbType.VarChar).Value = physicianNPI;
        cmd.Parameters.Add(new SqlParameter("@SearchFn", tbPreAuthFname.Text));
        cmd.Parameters.Add(new SqlParameter("@SearchLn", tbPreAuthLname.Text));
        cmd.Parameters.Add(new SqlParameter("@SearchAddress", tbPreAuthAddress.Text));
        cmd.Parameters.Add(new SqlParameter("@SearchDOB", tbPreAuthDOB.Text));
        cmd.Parameters.Add(new SqlParameter("@SearchSSN", tbPreAuthSSN.Text));
        cmd.Parameters.Add(new SqlParameter("@SearchDL", tbPreAuthDL.Text));
        cmd.Parameters.Add(new SqlParameter("@SearchPhn", tbPreAuthPhone.Text));
        cmd.Parameters.Add(new SqlParameter("@SearchBillingId", tbPreAuthAccountNum.Text));
        cmd.Parameters.Add(new SqlParameter("@SearchSex", tbPreAuthSex.Text));


            SqlDataAdapter sqlDA = new SqlDataAdapter(cmd);
            return sqlDA;
      }

      //------------   Patient Search Tab
      private String ConnString
      {
            get
            {
                  //HDS ConnectionString comes from the Web.Config
                  return ConfigurationManager.ConnectionStrings["123121ConnectionString"].ConnectionString;

            }
      }

      //------------   Patient Search Tab
      private String SearchString
      {
            get
            {
            String srchCmd = "SELECT PATIENT.MPI, PATIENT.LAST_NAME, PATIENT.FIRST_NAME, PRIMARY_INSURANCE.PLAN_NAME, PATIENT.DATE_OF_BIRTH, PATIENT.SEX FROM PATIENT LEFT JOIN PRIMARY_INSURANCE ON PATIENT.MPI = PRIMARY_INSURANCE.MPI WHERE PATIENT.LAST_NAME = @SearchLn OR PATIENT.FIRST_NAME = @SearchFn";
                  return srchCmd;

            }
    }



    //------------   PreAuthAdapter
    protected SqlDataAdapter CreatePreAuthAdapter()
    {

        SqlConnection conn = new SqlConnection(ConnString);
        SqlCommand cmd = new SqlCommand(PreAuthSearchString, conn);
        SqlDataAdapter sqlDAPreAuth = new SqlDataAdapter(cmd);
        return sqlDAPreAuth;
    }

   

    //------------   PreAuth Search Command
    private String PreAuthSearchString
    {
        get
        {
            String srchCmd = "SELECT PRE_AUTH.MPI, PRE_AUTH.INSURANCE_NAME, PRE_AUTH.PRE_AUTH_NUM, PRE_AUTH.EXPIRY_DATE, PRE_AUTH.VISITS FROM PRE_AUTH";
            return srchCmd;

        }
    }