When adding the standalone ESXi host to Veeam Backup & Replication, if you receive the error:
VMware ESXi with name '<hostnme/fqdn/ip>' already exists.
Please try an alternative method to connect to the server. For instance, if you attempted to add the ESXi host using its hostname/FQDN, try using the ESXi server's IP address instead. Conversely, if you initially used the IP address, add the ESXi host using its hostname/FQDN. If the DNS functionality is not operational during the attempt to add the standalone ESXi by its hostname, create a temporary hosts file entry for that alias on the Veeam Backup & Replication server and any VMware Backup Proxy that will be utilized for the restore.