I recently had a situation in Umbraco where I wanted to add a “Hostname and Culture” setting but when I clicked “save” I kept getting the error “Domain has already been assigned”. This was in Umbraco 7.2.8
After much searching on Stack Overflow where people were suggesting that I hack the domains table I decided against it since I’m using a Production system with no access to the Live database and no easy way of recovery without filling out oodles of paperwork to get Operations to undo my errors – embarrassing to say the least.
Finally, I realised that Umbraco wasn’t lying to me. The domain name was being used. I had a previous content page which was similar to the one I was editing and it used the domain name but it had been deleted. It was sitting comfortably in the “Recycle Bin” and when I removed it from there my problem was solved.
Lesson learned – clean up your “Recycle Bin”!
UPDATE – It seems that Umbraco 7.5.4 has an improved error message that indicates where the duplicate is.