using System;

using Newtonsoft.Json.Linq;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public class Payment

    {

        public int AccountId

        {

            get;

            set;

        }

​

        public Payment(int accountId) => AccountId = accountId;

    }

​

public static IEnumerable<Payment[]> GetBatches(IEnumerable<Payment> source, int count)

{

   var hashset = new HashSet<int>(count);

   var batch = new List<Payment>(count);

   var leftOvers = new Queue<Payment>();

​

   while (true)

   {

      foreach (var item in source)

      {

         // check if batched

         if (hashset.Add(item.AccountId))

            batch.Add(item); // add to batch

         else

            leftOvers.Enqueue(item); // add to left overs

​

         // if we are at the batch size start a loop

         while (batch.Count == count)

         {

            yield return batch.ToArray(); // return the batch

​

            batch.Clear();

            hashset.Clear();

​

            // check the left overs

            while (leftOvers.Any() && batch.Count != count)

               if (hashset.Add(leftOvers.Peek().AccountId)) // check the batch

                  batch.Add(leftOvers.Dequeue());

               else break; // we still have a duplicate bail

         }

      }

​

      if(batch.Any()) yield return batch.ToArray();

​

      if (!leftOvers.Any()) break;

​

      source = leftOvers.ToList(); // allocation  :(

      hashset.Clear();

      batch.Clear();

      leftOvers.Clear();

   }

}

​

    public static void Main()

    {

        var list = new List<Payment>()

        {new(1), new(2), new(3), new(4), new(4), new(5), new(6), new(4), new(4), new(6), new(4)};

        var batches = GetBatches(list, 3);

        foreach (var batch in batches)

            Console.WriteLine(string.Join(", ", batch.Select(x => x.AccountId)));

    }

}