Adding a reply-to sometimes causes a dead-end screen.

## Describe the bug

Entering a duplicate of an existing reply-to address leads users to a dead-end screen.

## [Bug Severity](https://docs.google.com/document/d/1tcD32bjJRaREsMgaUEoXAk400y1dKzzaKR0G5Sr1w20/edit)
See examples in the documentation

SEV-4 Low

## To Reproduce

Steps to reproduce the behavior:

1. On the reply-to address form
2. Have at least one existing reply-to address
3. Try to add the same reply-to address as a duplicate
4. Receive an error message
5. Notice you're stuck on that page. (See screenshot)

## Expected behavior

Serve the same error message in an error banner, and let the user fill out the form and resubmit. (See proposed interface)

## Impact

Describe overall user/system impact to consider when prioritizing this issue.

If applicable

Impact on Notify users: Its a dead-end. They can only navigate back or to a different section. They can't retry the form right away to fix their mistake

Impact on Recipients: None

Impact on Notify team: None

## Screenshots


## Proposed change


No implementation files or tests are named. Start by locating the reply-to address form and its duplicate-address error handling, then reproduce the dead-end behavior described in the issue. Done means the same error appears in an error banner while the form remains available for correction and resubmission.

Written by the indexing model from the issue text.