Do not create additional alert todo for already-assigned user

What does this MR do?

When an alert is assigned to a user, a todo is created for them. If another api request is sent which assigns them (for example, in the scenario of multiple reviewers), we would create an additional todo for the assignment, even if one already existed for the user.

This bug isn't entirely user-facing. Until multiple reviewers are supported, it's unlikely to be seen.

The main circumstance:

• Open the alert in two tabs
• Assign the alert to yourself in one tab
• Assign the alert to yourself in another tab
• See the double todos

I'm not sure if this warrants a changelog entry or not?

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team