For folks wondering why you get double-notifications for SMS messages, the FAQ mentions that "Join, by default, sends a notification for each SMS message you receive. You can either disable those under SMS settings, or disable your SMS app’s notifications".
However, I didn't realize at first that it's referring to the "SMS and Call Devices" toggle. The way it's currently worded, "Select to which devices SMS messages and phone calls on this device are sent" I expected that unchecking the option would stop SMS notifications completely, regardless of other settings.
Instead, unchecking stops the notifications specifically created by Join (not the SMS app itself). If your SMS app is checked under "Notification Apps", you'll still get those notifications and be able to interact with them as normal (reply, mark as read, etc.)
tl;dr, Clarify that unchecking "SMS and Call Devices" will stop the duplicate notification that Join sends out.
Maybe something like:
"By default, Join sends a notification for each SMS message or call you receive. Uncheck devices to disable."