Remove duplicit events from burndown data

What does this MR do?

It's possible that an event is followed by another event with the same action - typically if both merge request and commit mention Closes #issue_number, then two close events are created for the issue. We need to filter out these duplicit events because burndown chart uses these events to get number of open issues during time.

Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/12328

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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