private static List<T> RemoveDoubleItems<T>(List<T> list)
{
    List<T> newList = new List<T>();
    Dictionary<T,string> keyList = new Dictionary<T,string>();

    foreach (T item in list)
    {
        if (!keyList.ContainsKey(item))
        {
            keyList.Add(item, string.Empty);
            newList.Add(item);
        }
    }

    return newList;
}

private static List<T> RemoveDoubleItems<T>(List<T> list)
{
    return list.Distinct().ToList();
}