Radarr/NzbDrone.Common/IEnumerableExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace NzbDrone.Common
{
    public static class EnumerableExtensions
    {
        public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            var knownKeys = new HashSet<TKey>();

            return source.Where(element => knownKeys.Add(keySelector(element)));
        }
    }
}