C# 3.0: Need to return duplicates from a List<>

Question

I have a List<> of objects in C# and I need a way to return those objects that are considered duplicates within the list. I do not need the Distinct resultset, I need a

Answer 1

IEnumerable GetDuplicateColors(List cars)
{
    return cars.Where(c => cars.Any(c2 => c2.Color == c.Color && cars.IndexOf(c2) < cars.IndexOf(c) ) );
}    

It basically means "return cars where there's any car in the list with the same color and a smaller index".

Not sure of the performance, though. I suspect an approach with a O(1) lookup for duplicates (like the dictionary/hashset method) can be faster for large sets.