importing additional images into a project

hi

i have four folders in my 'pictures' folder in finder which i have imported into aperture as projects. from time to time i will add images to these folders, and would like the projects in aperture to update accordingly. the only method i can think of for doing this is to reimport the folder into the project, but then i get duplicate images for all of the images that i have previously imported into the project. is there a way of not importing duplicates, or of linking the project to the folder so that the projects can be kept in sync with the folders? this isn't an issue for importing photos from my camera as i can specify the project and the folder the images are to go in. but i also need to import other images from my local drive, and can't find a way to do this.

thanks

nick