Allow duplicate includes in .gitlab-ci.yml

What does this MR do?

Closes #63982 (closed)

This MR removes the error thrown when duplicate includes are included in a .gitlab-ci.yml file. Duplicate includes will be ignored.

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

If cross-browser testing is not required, please remove the relevant item, or mark it as not needed: [-] -->

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.