VoIP Configuration - Redundant Servers Both Registering To Cisco CM

I’m having an issue where my redundant pair of servers are both trying to register to Cisco CM. Cisco CM sees two distinct IP addresses trying to use the same account/username so they keep going in and out of a registered state in CM. I was able to put a network block up, so the backup server could not reach CM. This made the primary stable but is only a stopgap. How has anyone dealt with this?