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!
There seems to be a problem viewing the forum thread requested. If you'd like the boring old technical reason for the error this is it:
"The thread you are attempting to browse is deleted. We recommend you return to Who Am I? and browse the other threads there."