Plugin API usage

Hello.
We are developing plugin https://ww.wp.xz.cn/plugins/popup-by-supsystic and we have integration with your plugin there (you can subscribe from our PopUp plugin – to your MailPoet plugin). We are using method WYSIJA_help_user::addSubscriber() to add subscribers, but this method return true in case if subscriber already exists – in this place – http://prntscr.com/c9t2ha . So, after all check is done, and if subscriber do not need to be added – it still return true. Please tell me – can you fix this – to make it return false in this case?

https://ww.wp.xz.cn/plugins/wysija-newsletters/