From what I understand, you're opening a link from twitter, going back to twitter to open a different link correct? If so, how are you navigating back to twitter? If you're hitting the back key from IE, then the first link isn't being replaced, it's being closed. Hitting back on a tab in IE that cannot go back any further will close that tab. If you're using the task switcher or relaunching twitter, two tabs will be there.
Sent from my Lumia 920 using Tapatalk