Here are a couple of tricks to remove duplication using LINQ.

Firstly, for simple lists:

var numbersWithDuplicates = new[] { "One", "Two", "Two", "Three", "Four" };
var numbers = numbersWithDuplicates.GroupBy(x => x).Select(group => group.First());

…and secondly for more complex objects:

public class Person
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
}

…you can de-duplicate using any property you like:

var peopleWithDuplicates = new[]
{
    new Person { Id = 2, Name = "Alice", Age = 29 },
    new Person { Id = 1, Name = "Bob", Age = 30 },
    new Person { Id = 3, Name = "Claire", Age = 31 },
    new Person { Id = 4, Name = "Bob", Age = 50 },
};
var people = peopleWithDuplicates.GroupBy(x => x.Name).Select(group => group.First());