程序打开 9 个消息框，而我只想要 1 个

当我的计时器达到秒数（10、20 或 30）时，我希望出现一个消息框。这有效，但不是只有 1 个消息框，而是出现了 9 个！我不知道为什么？

    private void timer1_Tick(object sender, EventArgs e)
    {
        int hrs = sw.Elapsed.Hours, mins = sw.Elapsed.Minutes, secs = sw.Elapsed.Seconds;
        label5.Text = "";
        if (mins < 60)
            label5.Text += "0" + mins + ":";
        else
            label5.Text += mins + ":";
        if(secs < 60)
            label5.Text += secs;  
       else if(secs < 60)
            label5.Text += secs;
        if (comboBox1.Text == "10 seconden") // maximale tijd per beurt instellen.
            if (mins == 00 && secs == 10)
                MessageBox.Show("Je tijd is op!");
        if (comboBox1.Text == "20 seconden")
            if (mins == 00 && secs == 20)
                MessageBox.Show("Je tijd is op");
        if (comboBox1.Text == "30 seconden")
            if (mins == 00 && secs == 30)
                MessageBox.Show("Je tijd is op");