Exclude duplicates from emails on push recipients

What does this MR do?

It excludes duplicates from the recipients list before sending an email for the emails on push service.

There is another MR to clean up the list from duplicates before saving it to the database: !55550 (merged)

Related issue: #268207 (closed) the issue is confidential, but it was decided that this doesn't need a security MR.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done