If I understand correctly, you're trying to change the FQDN on one interface to value that currently has the second interface, is that correct? If I'm wrong, please provide some examples of DNS name and Domains you're using to better illustrate what you're trying to achieve. There are some limitations in validations for which we don't have currently a good solution in WebUI, you can try using API to modify interface that runs validations separately.

That is correct. I have found this behavior is still the same with 1.8.2. For some reason upgrading from previous version (1.6.x) to 1.8.x resulted in the wrong interface being marked as "Primary". On most of my systems the "Primary" interface is public facing and is what sets the FQDN. The "Provision" interface is private and in most cases has no hostname associated, just an IP. So far the only way to put the FQDN on the correct interface is to give it a fake FQDN. So if the host was "web0.tamu.edu" I have to give it the host name "web0p" and associate it to domain "tamu.edu". Then before saving I have to remove the host name and domain association on the non-primary interface. Once saved I have to go back and remove the "p" from the hostname so the system is correctly called "web0.tamu.edu".

The rename operation isn't a huge problem in this environment as I don't yet manage DNS with Foreman. I do one day hope to manage DNS and can see this being an issue where Foreman would start creating DNS and/or DHCP entries for this fake name that is only used to work around the validation.

I also am seeing this issue when using discovery. It sets my primary interface correctly, and then when I configure a network bridge and the new primary interface is br0 I can't update it. Foreman says the name is already taken but I can't remove the name from the previous primary interface. If I try to remove the name from one interface (e.g eth0) and move it to the other (e.g br0) same error. If I try to remove the name first I get a "No Route matches".