No, it should not cause more reposts. Older posts of the same topic are no longer in the search results, but all the same topics are there. So people will find the already posted stuff, but more easily. If you searched for some girl with the old search you will always find the same girl with the new one, but only once/topic... not tens of results but from one same topic.
Searching links is easier or harder - it depends. Let's say there used to be one topic containing lots of links to a site called "examplesite.com". The old search would return all the posts that have that link, so you could click every post one-by-one and look at the link. But who would like to do so, since why not browse thru the topic itself since it contains that many links? Jumping back & forth for because of some particular site only serves a purpose of finding illegal content of particular file sharing site... don't really know any other reason.
The new one would only return the latest link of the topic containing the search word. However, if the same "examplesite.com" would be on more then 1 topic, the new search would still return all the same topics that the old one.. with a link to the latest post with that result. So now we prefer giving out more different search results (=topics), just like Google, then giving out dublicate results from the same topic. So the new search is no longer interested in finding all links from one topic, but finding the topic.
It might sound confusing, but now have a search that functions much more like the search in the popular vBulletin software... only with that exception that ours is better, since it links to particular post, not returning only a list of topics. Other difference is that we show an except from the post.