Reg Creating Sessions

I have a job application form that includes 4 pages. The problem is for example when a user fills up 2 pages and if comes back by hitting "Back" button, and changes/updates the form, and then hits the "Continue" button, the form is creating a new id after hitting the continue button. So it is creating multiple records for the same user creating multiple ID's. I kind of know the solution, that if i create a session, the problem will be taken care of. But don't know how to create session.