I think we should leave it as it is. Adding a link to a snap related to it not only brings additional content such as another source, but also different comments from people here. Being allowed to just add outside links means that you would lose on the content of the comments from Snapzu.
To be honest I don't mind reposts, especially with the amount of new people trickling in every week, they will not have had the opportunity to see the old posts. If you know it's a repost and it annoys you, downvote. If you don't care, don't vote.