Question

With the following data

string[] data = { "a", "a", "b" };

I’d very much like to find duplicates and get this result:

a

I tried the following code

var a = data.Distinct().ToList();
var b = a.Except(a).ToList();

obviously this didn’t work, I can see what is happening above but I’m not sure how to fix it.

Answer 1

When runtime is no problem, you could use

var duplicates = data.Where(s => data.Count(t => t == s) > 1).Distinct().ToList();

Good old O(n^n) =)

Edit: Now for a better solution. =)
If you define a new extension method like

static class Extensions
{        

    public static IEnumerable<T> Duplicates<T>(this IEnumerable<T> input)
    {
        HashSet<T> hash = new HashSet<T>();
        foreach (T item in input)
        {
            if (!hash.Contains(item))
            {
                hash.Add(item);
            }
            else
            {
                yield return item;
            }
        }
    }
}

you can use

var duplicates = data.Duplicates().Distinct().ToArray();