If you already have a section break, you are actually seeing two first page headers, one for each section. These two first page headers are linked, meaning that they have the same content. You will have to go to the second header and click to deselect "Link to previous" on the Header & Footer Tools Design tab. After that you can change the headers independently.

Alternatively, delete the section break—but note that you will lose section formatting in the process (such as header/footer content), and that formatting must be recreated.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional