I have three different computer configurations with the identical Network Manager problem. I have a home wi-fi unit named ATTXXX. Each computer has one internal wi-fi interface and one external USB device. The internal wi-fi chips differ, but all the USB devices are Alfa AWUS036H. Two computers run Ubuntu 14.04; one is a Raspberry Pi 3 running Ubuntu Mate 16.04.
The problem occurs upon startup. The Network Manager establishes connections from both the internal and external devices to ATTXXX. Of course, nothing good happens until I open the Network Manager and disconnect one of the two devices.
Is there any cure for this?
Go to "Edit Connections..." in the network icon menu.
Find the wireless network name that you're connecting to, select it, and click the Edit button.
In the Wi-Fi tab, look at Device field, and it'll be blank right now, meaning that any interface can use that wireless network. Use the pop-up arrow at the end of the Device field to select the mac address of the interface that you DO want to use to connect. Other devices won't connect now, unless you create another script with their specific mac address in the Device field.