Allow creating duplicate frontend environment from GitHub branch, which is already used

**Is your feature request related to a problem? Please describe.**
I need creating two separate cloudfront distributions from the same git branch. That's because the logic of backend considers hostname, and it will return different data for different domains. I can not reuse the same CloudFront distribution, because of caching issues. Adding a Host header to cache key will lead to APIGW SNI violations, which can be solved with custom domain mappings. But to make them work with pull request previews, I need to include them in Cloudfront template, which is equivalent to just duplicating a distribution there, and a lot of work out of nothing.

**Describe the solution you'd like**
I'd like to create a duplicating frontend environment (connect the same branch for a second time).

**Describe alternatives you've considered**

- It is currently possible by duplicating the amplify application, but it looks too ugly.
- I can also duplicate the git branch, but what the heck? Why should I change gitflow to address UI issues of a CI server?

The issue gives no files, tests, or entry points. Start by tracing frontend environment creation and the validation that prevents reusing a GitHub branch; compare the requested duplicate environment with the CloudFront and pull-request-preview behavior described. Done means two environments can connect to the same branch without changing gitflow, while preserving distinct distributions and preview support.

Written by the indexing model from the issue text.