public class UniqueList<T> : List<T>
    {
        public new void Add(T item)
        {
            if (!this.Contains(item))
                base.Add(item);
        }
        public new void AddRange(IEnumerable<T> collection)
        {
            foreach (var e in collection)
                this.Add(e);
        }
        public new void Insert(int index, T item)
        {
            if (!this.Contains(item))
                base.Insert(index, item);
        }
        public new void InsertRange(int index, IEnumerable<T> collection)
        {
            int offset = 0;
            foreach (var e in collection)
            {
                if (!this.Contains(e))
                {
                    this.Insert(index + offset, e);
                    offset++;
                }
            }
        }
    }