Done this once or twice myself - last time when Lord of the Rings was on in 2001. Strictly speaking no, but once you've passed the foyer into the screen area and your ticket's been taken, there aren't ever any staff 'on the door' at each screen to check what you've actually paid to watch to prevent multiple viewings.
Heck, there are never any staff available to walk into the screens periodically to tell the local muppets to shut up talking during the film!
As long as you discreetly avoid any staff, you'll get away with it.