mattez this is happening due to a lack of UI feedback.
When you click Save for multiple domains, your browser fires off API request to create the domains.
If there's a lot of domains the request can take a few seconds or longer, so it sort of looks like your browser is doing nothing.
If you click Save again at that point, it will fire off the request again and you'll see that error because some of the domains have already been created.
We'll add some UI feedback like a modal progress indicator in a future update which should solve this.