XF 1.5 Reimport Threads and Nodes

We are doing a conversion from vBulletin 4.2.1 to Xenforo. A few month ago, we installed the new Xenforo and did an import from vBulletin. On this development Xenforo install, we have been adjusting settings, changing permissions, etc. We are nearing the point of going live with Xenforo, so we need to do another import in order to get the last several months of threads, posts, conversations, etc. from the live vBulletin forum. I've read your instructions, and it says that doing a second import will cause the following things to be duplicated: forums, threads, posts, avatars, and attachments. Is that the full list of what will be duplicated? Is there anything that can be done to not duplicate posts or nodes? What we would like to do is overwrite posts and/or nodes that already exist. Is this possible in any way?