If you've ever deleted an old project, grabbed the latest version from your source code repository and tried to import it you've probably seen the "Some projects cannot be imported because they already exist in the workspace" error. The key here is to get Eclipse to do a refresh, but you have to do it from the correct place. Just using the File menu and selecting Refresh won't do it.
Click Next and in the Import dialog, uncheck the Copy projects into workspace (I'm assuming you checked out from your source code control directly into a folder inside your workspace). Here is where you click the Refresh button. Once you've clicked the error should go away and you should be able to import the project.