I've just got an android phone (Nexus 5X). I have setup Google Voice to send/receive SMS in Hangout. Under the "Phone" menu in Google Voice, the option to "Receive text messages on this phone" is grayed out and it says "Receive text messages on this phone - disabled since you've opted in to send and receive SMS in Hangouts" next to it.
However, when I get SMS on my hangout, the SMS also shows up in the Messenger app. Is this because Messenger app is just pulling in all SMS message receive on the device? Is there a way to prevent Hangout SMS (GV text forwarded to it) from showing up in the Messager app?