I used to think this wasn't possible, but after a recent move, I had a set of speakers that showed up as R+? (or maybe L+?) which hadn't been set up as a stereo pair ever. What I ended up doing was trying to separate the stereo pair in the controller app, and after doing so, the "phantom" speaker disappeared.
I suspect your situation isn't the same, i.e. probably not a stereo pair. So, I'd try a couple of things, and let's see if someone from Sonos chimes in, or another community member.
I'd try unplugging the speaker (the real one!) and then rebooting your router. Once the router comes back up, go ahead and plug in that speaker to power again, and check in the controller to see if the "phantom" still appears.
Hmmm, I wonder what would happen if you tried to stereo pair them.
Finally, I'd recommend running a diagnostic and posting the number here, there might be data in that which would help Sonos get an idea as to how this happened.
Bruce