I visited your site and saw the double title. I could only reproduce the problem when pressing the register link on your forum page. When i use the pop login register link the problem doesn't show, so as far as i can see the problem isn't with the YJ module.
You might want to check the settings for the menu item concerned (show page title) and of the kunena forum.
For the second part of your question. YJ Pop Login isn't designed to show up at first visit. You might want to check out this: Popups & iFrames - Joomla! Extensions Directory
, there you can find modules designed to do what you are after.