using System;

using System.Linq;

using System.Collections.Generic;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        var list = new List<Example>

        {

            new Example {CatId = 1, Value = new { }},

            new Example {CatId = 1, Value = new { }},

            new Example {CatId = 1, Value = new { }},

            new Example {CatId = 2, Value = new { }},

            new Example {CatId = 2, Value = new { }},

            new Example {CatId = 3, Value = new { }}

        };

        var result = list.GroupBy(x => x.CatId)

            .SelectMany( g => g.Select( ( x, i ) => new { Item = x, rn = i + 1 } ) )

            .GroupBy(x => x.rn, x => x.Item);

        foreach(var x in result)

            Console.WriteLine(JsonSerializer.Serialize(x));

    }

}

​

public class Example

{

    public int CatId {get; set; }

    public object Value {get; set; }

}