For the last time
Shadow link: Created by the triforce of courage out of links shadow to test his worth
Dark link: Created by Ganon (and vaati that one time) to stop link in his various quests
The two are not the same. Dark link is a dark reflection of him while shadow link is his shadow. Dark link is evil aligned while shadow link is good.
I should mention that the hero of legend (LTTP, OOA, OOS and LA) learns the ability to create shadow links at some point and uses them to throw challenges at the new hero of legend (LBW) similar to the triforce of courage does for the hero of hyrule (Zelda and Zelda II)