@GetMapping("/{userId}/check_duplication")
	@PreAuthorize("hasAnyRole('A')")
	public Map<String, Object> checkDuplication(@PathVariable String userId) {
		User user = userService.getOne(userId);
		return buildMap(JSON_SUCCESS, user == null);
	}
Check if the given user id already exists.

@param userId userId to be checked
@return success json if true.
ngrinder-controller/src/main/java/org/ngrinder/user/controller/UserApiController.java