using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var originalData = new Data[]

        {

            new Data { ValueA = "Test 1" },

            new Data { ValueA = "Test 2" },

            new Data { ValueA = "Test 3" }

        };

        var newData = new Data[]

        {

            new Data { ValueA = "Test 1" },

            new Data { ValueA = "Test 4" },

            new Data { ValueA = "Test 2" },

            new Data { ValueA = "Test 4" }

        }; 

        var list = newData.Where(n => !originalData.Any(o => n.ValueA == o.ValueA))

            .GroupBy(data => data.ValueA.ToLower())

            .Select(data => data.First());

        foreach (var data in list)

        {

            Console.WriteLine(data.ValueA);

        }

    }

}

​

public class Data

{

    public string ValueA { get; set; }

}