Update header guidelines to avoid duplicates (github#34030)

contributing/content-style-guide.md

@@ -130,7 +130,7 @@ Workflow runs are delayed when too many workflows run at once. Since many users
 Use H2 for headers, and H3 for subheaders. When referring to headers, surround the header name with quotation marks.
 - **Use:** Under “User licenses”, view your total licenses.
 
-Our guidelines for writing titles also apply to writing headers. For more information, see the [content model](/contributing/content-model.md#titles).
+Each header on a page must be unique. Our guidelines for writing titles also apply to writing headers. For more information, see the [content model](/contributing/content-model.md#titles).
 
 To orient readers and help them understand if the section is relevant to them, include introductory content after a header - don’t locate a subheader directly following a header.