When adding the standalone ESXi host to Veeam Backup & Replication, if you receive the error:
VMware ESXi with name '<hostnme/fqdn/ip>' already exists.
Use an alternate way to contact the server. For example, if you tried adding the ESXi host by its hostname\FQDN, use the ESXi server's IP address. If you tried using the IP address, instead, add the ESXi host by its hostname\FQDN. If for some reason DNS is not functional when attempting to add the standalone ESXi by its hostname, make a temporary
hosts file entry on the Veeam Backup & Replication server and use that alias.