This should put an end to the "debate" on the spelling of "Khyber Pakhtunkhwa" == 

 Link to the "Constitution (Eighteenth Amendment) Act, 2010" as notified in the Official Gazette of Pakistan from the Official Website of the National Assembly of Pakistan: 

  

 Please start with Clause 3 and go through entire legislation and you will see what the official and legal spelling are. Note also the table in Article 16(3). If this doesn't convince some of you guys, I don't know what will.    

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