document_completed eventhandle run 2 times

[GoogleCodeExporter]

What steps will reproduce the problem?
1. GeckoWebBrowser.Navigate(targetUrl)
2. GeckoWebBrowser.Document_Completed += new EventHandle 
(myBrowser_DocumentCompleted)
3. void myBrowser_DocumentCompleted(object Sender, EventArg e)
{
    textBox1.Text += "just appear One time, why Two?? /n"
}

What is the expected output? What do you see instead?
I want the textBox1 just have 1 line of text, but there were 2, why? it's mean 
there are two document_completed event?

What version of the product are you using? On what operating system?
i use Geckofx15

Original issue reported on code.google.com by autumn....@gmail.com on 10 Dec 2012 at 7:46

[GoogleCodeExporter]

Sometimes, even with basic Browser, document completed gets fired multiple 
times. websites like facebook.com, can fire it multiple times

Original comment by SiscoDe...@gmail.com on 15 May 2015 at 12:07