steps to reproduce: 1. Log in as an admin and load https://marketplace-dev.allizom.org/en-US/developers/app/backpack/edit 2. Change the manifest URL to https://people.mozilla.org/~kraj/test-files/webapps/webapp-with-de-as-default-locale.webapp 3. Save changes expected behavior: If we try to replace a manifest URL with another which is linked to a different app, we should throw an error actual behavior: Changes are saved.
https://github.com/mozilla/zamboni/commit/f9267d0 Verifying the duplicate app domain depends on the waffle switch "webapps-unique-by-domain".