I ran into the same issue recently, the secondary manager ip had been set in a previous version (possibly 6) and was never removed when we implemented MDR I assume.
Support advised that this was probably a bug and sent it to development.
The only work around they gave me was to perform a resetconfig on the sensor and then reinstall it into the manager.
I had asked them if breaking the MDR pair would allow me to run the command but they were unsure.
They did tell me there was no impact in having this value set and that it should be fixed in a future release.
Thanks for the response. Unfortunately, I don't currently have physical access to this sensor, so a resetconfig is going to take me down. I may try to break the MDR pair to fix it. Will report back if I do that.
Running 'deinstall' should trick the sensor into believing there is no MDR pair, which should allow you to remove the second IP address. Just run 'setup' or 'set sensor sharedsecretkey' to reestablish trust with the manager.