Hi,
There are so many ways you can prevent by user to multiple submitting. That is depend on you. I have give some explanation what i think at right now in my mind.
1 ) Make one session variable which contains just any value when you first time submit process transaction set that session variable null. Now at your process transaction business logic page check if that session variable contains value null do not process ahead.
2 ) You can make your button disable when process transaction.
3 ) See link
http://jquery.com/demo/thickbox/ which is example of thick box. you can add something like this when you process transaction and just see process images using thick box...