I wouldn't call this a MAJOR iLife 06 Problem, more 'a small bug'.
iTunes has done this for me once or twice in importing every song into the library, but showing them all twice. because of not using garageband, or having it out here in poland with me, i can't offer any advice, but i wish you the best of luck finding out.
if there is a delete option it may be possible to automate it, but it may require a bit of custom applescripting.
maybe you should check to see that it hasn't actually installed everything twice