using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<int> motherlist = new List<int> { 1, 1, 2, 5, 7, 2, 2, 2, 6, 1 };

        List<List<int>> children = motherlist.DistributeDuplicates();

        Console.WriteLine(string.Join(Environment.NewLine, children.Select(c => string.Join(",", c))));

    }

}

​

public static class EnumerableExtensions

{

    public static List<List<T>> DistributeDuplicates<T>(this IEnumerable<T> seq, IEqualityComparer<T> comparer = null)

    {

        if(comparer == null) comparer = EqualityComparer<T>.Default;    

        Dictionary<T, int> dupCounter = new Dictionary<T, int>(comparer);

        foreach(T item in seq)

        {

            bool isDup = dupCounter.TryGetValue(item, out int count);

            dupCounter[item] = isDup ? count + 1 : 1;

        }

        int maxDupCount = dupCounter.Values.Max();

        var resultList = new List<System.Collections.Generic.List<T>>(maxDupCount);

        for(int i = 0; i < maxDupCount; i++)

        {

            List<T> subList = dupCounter.Where(kv => kv.Value > i).Select(kv => kv.Key).ToList();

            resultList.Add(subList);

        }

        return resultList;

    }

}

​