Hi Eric,
I did have a look into the settings of Shopware 6 and Shopware 5 and yes indeed it is right, that you can not define the same URL for different languages/locales at the moment whereas in Shopware 5 this was possible because there was no validation on those domains.
I opened an issue tracker ticket concerning this specific feature:
https://issues.shopware.com/issues/NEXT-7086
Updates to come to the ticket directly!