i believe if you have watched the video, the professor said the cashed route will only consider one path whichever reaches to the target first. In the video, professor said we will consider the path sefjd is because she "assume", sefj route reaches "D" first before "scgk",notice if both rreq reaches at d at the same time, it will cause collision. 

rrep is the "unicast", there should exist only one rrep of the route between the source and target. every possible route arrive at target d, the rreq packet will be discarded. once S receive the rrep the path sefjd will be cahsed, but scgkd will not.