[FIX] Fixed sending of emails to only new newsletter-subscribers.

[johnson-oragui]

Describe the bug
Users who have already subscribed to the newsletter receive emails of newsletter confirmation when they subscribe again. This is causing unnecessary email clutter and potentially impacting deliverability rates.

To Reproduce
Steps to reproduce the behavior:

1. Subscribe to the newsletter using the provided form.
2. Re-subscribe to the newsletter using the same email address.
3. Verify that multiple copies of the newsletter are being sent to the subscribed email address.

Expected behavior
Users should only receive the first email on first subscription regardless of how many times they subscribe.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• Browser brave
• Version 22

Solution:

• Update Newsletter Sending Logic:
  Modify the newsletter sending logic to only send emails to subscribers with a single entry in the database.

Additional context

• Error Handling:
  Implement error handling to gracefully handle cases where a user attempts to subscribe with a duplicate email address.
• User Interface:
  Provide feedback to the user if their subscription attempt is unsuccessful due to a duplicate email address.

Related Issue

newsletter bug