Java, Uniquely identifying several JAVA applets throughout several html pages

I'm developing a WEB site containing a number of JAVA applets covering
a number of HTML pages. Each of the applets will report to a central database
through JDBC. The question is, how can I know that each applet belongs to
the same user instance? What if the user opens two browsers on the same
machine? How do I get some sort of applet signature within a group of HTML
pages?
Any ideas?
Mat.
m...@cam.org