在c#中获取列表中重复项的索引

using System;
using System.Collections.Generic;

class Program
{
    static List<int> GetDuplicateIndexes(List<int> list)
    {
        List<int> duplicateIndexes = new List<int>();
        Dictionary<int, int> indexDict = new Dictionary<int, int>();

        for (int i = 0; i < list.Count; i++)
        {
            if (indexDict.ContainsKey(list[i]))
            {
                duplicateIndexes.Add(indexDict[list[i]]);
                duplicateIndexes.Add(i);
            }
            else
            {
                indexDict.Add(list[i], i);
            }
        }

        return duplicateIndexes;
    }

    static void Main(string[] args)
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 2, 6, 7, 3, 8, 9, 4 };
        List<int> duplicateIndexes = GetDuplicateIndexes(numbers);

        Console.WriteLine("Duplicate indexes:");
        foreach (int index in duplicateIndexes)
        {
            Console.WriteLine(index);
        }
    }
}