collector.ts

    public class Collector<TKey>
    {
        private HashSet<TKey> seen = new HashSet<TKey>();

        public IEnumerable<TData> CollectBy<TData>(IEnumerable<TData> input, Func<TData, TKey> extractKey)
        {
            foreach (var inp in input)
            {
                var key = extractKey(inp);
                if (seen.Contains(key))
                    continue;
                seen.Add(key);
                yield return inp;
            }
        }
    }