Xcode 4 adding images to a project
Files in the XCode directories don't necessarily have to be in the project.
For some reason, XCode doesn't like overwriting files, and that's the warning message that I usually get when this happens too - "Multiple errors occured."
What I suggest you do is remove all of the @2x files from your project directory, and add them back into XCode using "Add Files..." Make sure you select "Copy files into project directory if necessary" though, otherwise they will just be referenced to their current location on disk.