两个ObservableCollections的联合返回重复项

using System;
using System.Collections.ObjectModel;
using System.Linq;

public class Program
{
    public static void Main()
    {
        ObservableCollection<int> collection1 = new ObservableCollection<int> { 1, 2, 3, 4, 5 };
        ObservableCollection<int> collection2 = new ObservableCollection<int> { 4, 5, 6, 7, 8 };

        ObservableCollection<int> duplicates = FindDuplicates(collection1, collection2);

        Console.WriteLine("重复项：");
        foreach (int item in duplicates)
        {
            Console.WriteLine(item);
        }
    }

    public static ObservableCollection<int> FindDuplicates(ObservableCollection<int> collection1, ObservableCollection<int> collection2)
    {
        ObservableCollection<int> duplicates = new ObservableCollection<int>();

        foreach (int item in collection1)
        {
            if (collection2.Contains(item))
            {
                duplicates.Add(item);
            }
        }

        foreach (int item in collection2)
        {
            if (collection1.Contains(item) && !duplicates.Contains(item))
            {
                duplicates.Add(item);
            }
        }

        return duplicates;
    }
}