I assume you are talking about the main header where a logo or banner would be placed?  If so, the header you place within your skin file will appear on all pages within your portal if you apply that skin to each page.

 

It's the Title field under page settings.

You can leave it blank if you want it to be auto-generated.  If you set them all to exactly the same thing you are really hurting your search engine ranking.

If you really want to set them all to exactly the same thing, then putting the following code in your default .aspx page should work:

<% Title="My One and Only Title" %>

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