Loading a new window on url submission

hey folks,
i m developing a web app. the requirements states that when a person login from the homepage. the session (rest of web app) open in new window. but that leaves a problem. it leaves a copy of web app homepage after login in browser opened.that gives a bad impression having 2 copies of one paged open. now what i have in mind and is it possible that if “when someone enter a url if i put body onload function which trigger a new window?” i guess i will end in same problem.as two copies will open.right? can someone help me with this problem.

Edit:just saw a website which do sort of what i want. when u click enter. the page stays same but another window open with the website. but the page where it says enter stays the same. i don’t know JS much so can anyone help me out with this?