HasDistinct

public static class ListExtensions
{
        //Usage Example: person.Names.HasDistinct(x => x.Name);
        public static bool HasDistinct<Type, TProperty>(this IEnumerable<Type> obj, Expression<Func<Type, TProperty>> propertySelector)
        {
            IEnumerable<TProperty> listOfThatProperty = obj.Select(propertySelector.Compile());

            return listOfThatProperty.Count() == listOfThatProperty.Distinct().Count();
        }
}
public class UnitTest1
{
        [Fact]
        public void Test1()
        {
            PersonName personName1 = new PersonName() { Name = "Razali" };
            PersonName personName2 = new PersonName() { Name = "RuiRui" };

            Person person = new Person()
            {
                PersonNames = new PersonName[]{ personName1, personName2 }.ToList()
            };

            Assert.True(person.PersonNames.HasDistinct(p => p.Name));

            personName1 = new PersonName() { Name = "Razali" };
            personName2 = new PersonName() { Name = "Razali" };

            person = new Person()
            {
                PersonNames = new PersonName[] { personName1, personName2 }.ToList()
            };

            Assert.False(person.PersonNames.HasDistinct(p => p.Name));
        }
}