I had this happen with my first import into iPhoto 4 as well. What I realized was that I had imported pictures from my camera in iPhoto 2, forgotten to erase the camera afterwards, then taken more pictures. When I imported into iPhoto 4, it did not ask if I wanted to skip the duplicates. After clearing the duplicates manually, I cleared my camera and took one test picture. I imported it once, disconnected the camera, and imported again. This time it did ask if I wanted to skip the duplicate. So, I wonder if it had something to do with the transition from iPhoto 2 to iPhoto 4. I have not tested again, so I don't know if the problem is permanently gone.
As a side note, iPhoto 4 shows a picture of my actual digital camera instead of the generic camera icon. Bonus!