violation of primary key constraint

private void button2_Click(object sender, EventArgs e)
      {
string path = "E:\\BatchMaster.xls";

          string excelConnectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + path + ";Extended Properties=Excel 12.0;Persist Security Info=False";

          OleDbConnection excelConnection = new OleDbConnection(excelConnectionString);

          OleDbCommand cmd = new OleDbCommand("Select  *from [Sheet1$]", excelConnection);
          excelConnection.Open();
          OleDbDataReader dReader;
          dReader = cmd.ExecuteReader();
         SqlBulkCopy sqlBulk = new SqlBulkCopy(con);

         sqlBulk.DestinationTableName = "BatchMaster";
          sqlBulk.WriteToServer(dReader);
          excelConnection.Close();
          MessageBox.Show("Successful");


      }