我对FCM设备组有疑问，如果我使用ADD API在相同的设备组中添加相同的registration_id会发生什么情况？这是唯一的registration_id的键集？
示例：
Add REGISTRATION_ID_1 -> Group AAA
Result:
Group AAA [REGISTRATION_ID_1]
Add REGISTRATION_ID_2 -> Group AAA
Result:
Group AAA [REGISTRATION_ID_1, REGISTRATION_ID_2]
Add REGISTRATION_ID_1 -> Group AAA
Result:
Group AAA [REGISTRATION_ID_1, REGISTRATION_ID_2, REGISTRATION_ID_1]
或者我第二次添加REGISTRATION_ID_1时将产生类似“注册ID已经存在”的错误
谢谢！