C#:需要从List <>返回重复项

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 2, 3, 6, 7, 8, 9, 10, 1 };

        var duplicates = GetDuplicates(numbers);

        foreach (var duplicate in duplicates)
        {
            Console.WriteLine(duplicate);
        }
    }

    static List<T> GetDuplicates<T>(List<T> list)
    {
        return list.GroupBy(x => x)
                   .Where(group => group.Count() > 1)
                   .Select(group => group.Key)
                   .ToList();
    }
}

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