usb-picker: Print warning if multiple devices with the same id exist

[oberien]

Mention to add both by port.

[main--]

Can this even happen? Shouldn't the blacklisting take care of it right now in the wizard?

We don't really care a lot if you shoot yourself in the foot by editing the config.

[oberien]

It's still nice UX to double check when entering the usb-picker (i.e. when first listing the available usb devices) and print a warning if multiple devices with the same id are connected. And it's not heavy on the usb-picker.