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!
Random screenshots
looks good with that hole in the cieling
i second that
I third that.
I fourth that.
I- No. I will not fifth that.
I fifth that.
I-Must resist sixth-ing that
must resist
no
yes i must resist
I si-NO
I-I-I-NOOOOOOO
I give up
I sixth that!!!
Must/.................................... SEVENTH