Reimporting

I have a version 2.x instance of WordPress and I’m upgrading to 3.8.1 with a different instance. I did an export of my data from the old system and imported it to the new one. The problem is that I didn’t get the new one set up fast enough and there is now new content on my old site. If I export the old site and again import it into my new site, will I get double entries for what I’ve already imported? Or will the import be smart enough to only import what’s not already there?