Exclude by Sibling Condition

Considering the following XML fragment:
      
  <page name="p0" tab="products"/>
  <page name="p1" tab="products"/>
  <page name="p2" tab="products"/>
  <page name="d" tab="downloads"/>
  <page name="s" tab="support"/>
      
How can I select a subset that excludes elements with a tab attribute
value matching that of a previous sibling?
     
Meaning I want a list with no redundant @tab values like:
     
  <page name="p0" tab="products"/>
  <page name="d" tab="downloads"/>
  <page name="s" tab="support"/>
     
Thanks, 
Mike