Copying an Image from one Project to another (you can't put Images in Folders; Folders hold only Albums and Projects) you will create a new Original file, and you will end up with two originals and some difficulty down the road with versioning. The generally accepted workflow is to store your Images in Projects (horribly misnamed, btw -- imho the single worst interface decision in Aperture), and use Albums for doing work. Any Image can be any number of Albums. It simply 'shows' there, but 'lives' in a Project. You remove Images from Albums (and they stay in your Library). You delete Images from Projects (and they are moved to the Trash, which when emptied removes them from your Library).
You can drag an Image from any Project into any Album, and it stays in the Project. You can move any Image into any Project by dragging (and it is removed from the originating Project). And you can copy Images from one Project to another (as you've discovered), but doing so is mostly ill-advised.