Android Auto gives two notifications for new text messages.
This issue appears to be caused by Android Auto being unable to recognize Pulse SMS as the default SMS app. While this remains an issue, we can offer a solution to address the problem.
To ensure that you receive notifications exclusively from Pulse SMS, you may disable the SMS permission for Android Auto. This can be done by accessing your system settings, locating Android Auto in the installed apps list, and disabling the SMS permission.
Once you have completed this step, you will only receive notifications from Pulse SMS when new messages arrive.