在WinForm中，如何实现MDI窗体及防止重复打开子窗体

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace EC_Tool
{
    public partial class MainForm : Form
    {
        public MainForm()
        {
            InitializeComponent();
        }

        private void Main_Load(object sender, EventArgs e)
        {
            this.Text = "EC-Tool";
            this.WindowState = FormWindowState.Maximized;
            this.IsMdiContainer = true;
        }

        private void menuStrip1_ItemClicked(object sender, ToolStripItemClickedEventArgs e)
        {
            if (e.ClickedItem.Name == "參數設置ToolStripMenuItem")
            {
                var form = new ParameterForm();
                ShowMdiChildren(form);
            }
            else if (e.ClickedItem.Name == "分類設置ToolStripMenuItem")
            {
                var form = new CategoryForm();
                ShowMdiChildren(form);
            }
        }

        #region 自定義方法

        /// <summary>
        /// 彈出MDI子窗口
        /// </summary>
        /// <param name="form"></param>
        private void ShowMdiChildren(Form form)
        {
            bool flag = true;

            for (int i = 0; i < this.MdiChildren.Length; i++)
            {
                if (form.GetType() == this.MdiChildren[i].GetType())
                {
                    this.MdiChildren[i].Activate();
                    flag = false;
                    break;
                }
            }

            if (flag)
            {
                form.MdiParent = this;
                form.Show();
            }
        }

        #endregion
    }
}