It is a non-linear map which have many paths to take and reach the end. The point is, that you play a map one time and you are happy. Then you have a little break in Portal, because there aren't any new released maps for some time. But the answer is here: play the Deja-Vu again, but choose a different path with different puzzles. You can choose freely the way that is going to lead you to the end. Thats it!
This Version is not entirely final. The sound/voice isn't added. To bad i can't get it to work and didn't have the time to take a look at it. The chapters...