using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var dpia100 = new List<string>()

        {

            "1234",

            "1234",

            "1234",

            "1234",

            "567",

            "567",

            "890"

        };

​

        var skipFirst =dpia100.GroupBy(row => row) // Group by same dpia100 records

                .SelectMany(groupedRows => groupedRows.Skip(1)); // Skip the first one -> This will skip all data that has been previously exported

        foreach (var row in skipFirst)

        {

            Console.WriteLine(row);

        }

    }

}