I am not sure whether Pocket, prevents users from adding duplicate articles to the library, I think it doesn't.
It would be really useful I think to add ability to check whether the url that is supplied with the article is not already in the pocket library, if it is, it should inform the user about it.
As my Pocket collection has grown, it is often quite hard to remember whether I have already 'bookmarked' this article or not. This should ideally avoid this.