Bug 80833 [Import/Export] Import exisiting should pre-check for project already in workspace
While it is good that it lets you import projects in the default workspace location, it should not let you select projects that already exist in the workspace (since importing such a project will fail). When searching for .project files in the file system, it could use IWorkspaceRoot.findFilesForLocation followed by checking IFile.exists() to filter out .project files for existing projects.
bundles/org.eclipse.ui.ide/src/org/eclipse/ui/internal/wizards/datatransfer/WizardProjectsImportPage.java
* Return null if this file does not exist or is not of valid format.
	 */
	protected TarFile getSpecifiedTarSourceFile() {
		return getSpecifiedTarSourceFile(archivePathField.getText());
	}
	/**
	 * Answer a handle to the zip file currently specified as being the source.
	 * Return null if this file does not exist or is not of valid format.
	 */
	private TarFile getSpecifiedTarSourceFile(String fileName) {
		if (fileName.length() == 0)
			return null;
		try {
			return new TarFile(fileName);
		} catch (TarException e) {
			displayErrorDialog(DataTransferMessages.TarImport_badFormat);
		} catch (IOException e) {
			displayErrorDialog(DataTransferMessages.ZipImport_couldNotRead);
		}
		archivePathField.setFocus();
		return null;
	}
	/**
	 * Display an error dialog with the specified message.
	 * 
	 * @param message
	 *            the error message
	 */
	protected void displayErrorDialog(String message) {
		MessageDialog.openError(getContainer().getShell(),