session destroy ??

In my online store i use session. I use a script to check that a user dont make 2 order during the same session using the back button for that i put his sessionid in the db and check it before processing the order. My probleme is how can i give a new session_id to the person when he goes back to the home page of my site so he can do a new order ? I try session_destroy , session_unset but the person have still the same session_id REMARK I always use the same string into session_name("abcdef") for all the user. Is it a problem? Thanks for your help