using System;

using System.Collections.Generic;

using System.Runtime.InteropServices;

​

public static class Program

{

    public static void Main()

    {

        List<int> list = new() {4, 5, 7, 3, 5, 4, 2, 4};

        Console.WriteLine($"Before: [{String.Join(", ", list)}]");

        int removedCount = list.RemoveDuplicatesByKey(x => x);

        Console.WriteLine($"After: [{String.Join(", ", list)}], Removed: {removedCount}");

    }

​

    /// <summary>

    /// Removes all the elements that have a key that appears more than once,

    /// according to a specified key selector function.

    /// </summary>

    public static int RemoveDuplicatesByKey<TSource, TKey>(this List<TSource> list,

        Func<TSource, TKey> keySelector,

        IEqualityComparer<TKey> comparer = default)

    {

        ArgumentNullException.ThrowIfNull(list);

        ArgumentNullException.ThrowIfNull(keySelector);

        Dictionary<TKey, int> occurences = new(list.Count, comparer);

        foreach (TSource item in list)

            CollectionsMarshal.GetValueRefOrAddDefault(

                occurences, keySelector(item), out _)++;

        return list.RemoveAll(item => occurences.TryGetValue(

            keySelector(item), out int value) && value > 1);

    }

}