Fixes a bug that caused duplicate sticky header rows to appear if multiple outline items start on the same row.

Sort of addresses #52722, although arguably the real issue there is that duplicate outline items are being created in the first place.

Before:

After:

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Fixed a bug that caused duplicate sticky header rows to appear if multiple outline items start on the same row.