Yes, you've got the general idea of double NAT. The primary purpose of NAT is to translate/map public IP(s) to private IP ranges (10.0.0.0/8 172.16.0.0/12, 192.168.0.0/16) to facilitate Internet access. For this reason, it should only be done once, probably at the provider edge (WAN). In reality you could do it as many times as you want, but future troubleshooting could be difficult and certain applications might not function properly.