Why does my iPhone keep on showing old pictures/videos as new when syncing with iPhoto?

I take a lot of photos and videos on my iPhone that I regularly import to iPhoto. My iPhoto is set up to “Hide photos already imported” and yet it’s still showing several old photos and videos that I’ve already imported. I’ve tried importing them again, but the next time I plug in my iPhone, they still show up as new items to be imported. Short of just deleting them permanently from my iPhone, does anyone have any idea how I can keep those old photos/videos from showing up as new on iPhoto?