fix multiple notifications from being sent for multiple labels

What does this MR do?

Prevent multiple notifications from being sent when multiple label (upon which a user is subscribed) are added to an issue.

Are there points in the code the reviewer needs to double check?

See the changes in the spec/support/email_helpers.rb

Why was this MR needed?

Multiple duplicate notification emails are sent when adding N labels upon which a user is subscribed. This is noise in the inbox.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

/labels ~Discussion notifications Closes #37691 (closed)