Is there an AddUnique method similar to Addrange() for alist in C#

One choice is to add them and remove the repeated ones:

var list = new List<Car>();
list.AddRange(GetGreenCars());
list.AddRange(GetBigCars());
list.AddRange(GetSmallCars());
list = list.Distinct().ToList();

Another option is to do something like:

public static void AddUnique<T>( this IList<T> self, IEnumerable<T> items )
{
    foreach(var item in items)
        if(!self.Contains(item))
            self.Add(item);
}


var list = new List<Car>();
list.AddUnique(GetGreenCars());
list.AddUnique(GetBigCars());
list.AddUnique(GetSmallCars());

A List<T> doesn't seem to be the appropriate collection here. You probably want an ISet<T> implementation such as HashSet<T> (or SortedSet<T> if you need ordering).

To allow this, you will need to write an IEqualityComparer<T> implementation that defines equality between cars according to the Name property. If this is the 'canonical' definition of car-equality, you can also consider directly building this definition into the Car type itself (object.Equals, object.GetHashCode and ideally implement IEquatable<T> too).