NewWindow event gets called twice for certain html href tags. For example, using the EO Tabbed Browser example, if you go to:
http://www.rapidtables.com/web/html/link/html-link-new-window.htm and click on the link that is under "Open a link in a new window with specified size" this calls the NewWindow event twice. If you click this link using Chrome, only one new popup comes up.
Interestingly enough, if you remove the target=popup attribute, it will only call NewWindow once with the JS popup information. However, it will also replace the original browser window with the location in the href="..".

For Reference, this is the html:
<a href="../html-link.htm" target="popup" onclick="window.open('../html-link.htm','name','width=600,height=400')">Open page in new window</a>