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
getFilesForProject(filesToImport, provider, importSource.toFile());
			ImportOperation operation = new ImportOperation(project.getFullPath(), 
					importSource.toFile(), 
					FileSystemStructureProvider.INSTANCE, this, filesToImport);
			operation.setContext(getShell());
			operation.setOverwriteResources(true);	
			operation.setCreateContainerStructure(false);
			operation.run(monitor);				
		}
		
		return true;
	}
	/**
	 * Return a list of all files in the project
	 * 
	 * @param provider
	 *            The provider for the parent file
	 * @param entry
	 *            The root directory of the project
	 * @return A list of all files in the project
	 */
	protected boolean getFilesForProject(Collection files,
			IImportStructureProvider provider, Object entry) {
		List children = provider.getChildren(entry);
		Iterator childrenEnum = children.iterator();
		while (childrenEnum.hasNext()) {
			Object child = childrenEnum.next();
			
			
			files.add(child);