So I’m rewatching The Vampire Diaries right? And I make it to the last episode of season 6. Throughout the episode, it shows future visions of everyone with Elena. In one of them with Damon, he’s the one to tell her that she was linked to Bonnie. This was the first time she had heard of it. But later after Kai is dead, everyone goes through the visions, which we now know the purpose of. Through all of their visions, Elena knew about the link with everyone. Damon was the last person to be in a vision with her.

So how did Elena know about the link when Damon was the one to tell her? He was the last one, so it would make no sense for her to know about it with everyone else, or to be surprised when Damon tells her.