@GetMapping("/{userId}/check_duplication")
	public Map<String, Object> checkDuplicationForRegistration(@PathVariable String userId) {
		checkTrue(config.isSignUpEnabled(), "Access to this url is not allowed when sign up is disabled");
		User user = userService.getOne(userId);
		return buildMap(JSON_SUCCESS, user == null);
	}
Check the user id existence.

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