DDLS-348: Amend the registration so that users can't sign up more than once

[Gugandeep]

Purpose

Briefly describe the purpose of the change, and/or link to the JIRA ticket for context

Fixes DDLS-348

Approach

Explain how your code addresses the purpose of the change

Learning

Any tips and tricks, blog posts or tools which helped you. Plus anything notable you've discovered about DigiDeps

Checklist

• I have performed a self-review of my own code
• I have updated documentation (Confluence/ADR/tech debt doc) where relevant
• I have added tests to prove my work
• The product team have approved these changes
• I have checked my work for potential security issues and refered to the OWASP top 10

Frontend

• I have run an in-browser accessibility test (e.g. WAVE, Lighthouse)
• There are no deprecated CSS classes noted in the profiler
• Translations are used and the profiler doesn't identify any missing
• Any links or buttons added are screen reader friendly and contextually complete
• If adding GA events, I have updated or checked the existing category or label values