If you attempt to add an FP sensor and it becomes partly added

[babiojd01]

You may receive the error message that the active peer already exists. To fix this you can ssh into your FMC device, locate the UUID by cd changing into /usr/local/sf/peers to see what peers exist. In my case only the IP address portion was there not the string of characters. You can then cd /usr/local/sf/bin/remove_peer.pl "IP or NAME" FORCE.

[Marvin Rhoads]

Good tip.

You can also check the peers via querying the database as described in this thread:

https://community.cisco.com/t5/firepower/fmc-unable-to-save-a-backup-or-to-upgrade/td-p/3094446

From the FMC cli, become root ("su -") and run this command:

mysql -padmin sfsnort -e "select name,ip,uuid,role from EM_peers where role !=0"

[SMP2]

Thanks for posting this up babiojd01, nothing is more bothersome than the lack of available solutions to Firepowers endless problems and not being able to find a solution from a quick search.