How do I concatenate two hashes in Python? Concatenating and double SHA-256 hashing

"a751efbeabe73bdf9d08df5760104feff915d9d807d4c62178cdeb98d8c25f43" 

with itself

"a751efbeabe73bdf9d08df5760104feff915d9d807d4c62178cdeb98d8c25f43" 

should output
15eca0aa3e2cc2b9b4fbe0629f1dda87f329500fcdcd6ef546d163211266b3b3


  
    
      import hashlib
      header_hex = "a751efbeabe73bdf9d08df5760104feff915d9d807d4c62178cdeb98d8c25f43a751efbeabe73bdf9d08df5760104feff915d9d807d4c62178cdeb98d8c25f43"
      
      header_bin = header_hex.decode('hex')
      
      hash = hashlib.sha256(hashlib.sha256(header_bin).digest()).digest()
      
      hash.encode('hex_codec')
      '2b4a9fdb97e89e73c4647791e476090eaad04f43c93a3de21a95d2c4fc8e8e0c'
    
  


I'm getting the wrong answer :/?
You have to byte-swap the hex values BEFORE concatenating them. You also have to byte-swap the double hashed hex string!


  
    
      import hashlib 
      header_hex = "435fc2d898ebcd7821c6d407d8d915f9ef4f106057df089ddf3be7abbeef51a7435fc2d898ebcd7821c6d407d8d915f9ef4f106057df089ddf3be7abbeef51a7" 
      
      header_bin = header_hex.decode('hex') 
      hash = hashlib.sha256(hashlib.sha256(header_bin).digest()).digest() 
      hash.encode('hex_codec') 'b3b366122163d146f56ecdcd0f5029f387da1d9f62e0fbb4b9c22c3eaaa0ec15' >>> 
      
      hash[::-1].encode('hex_codec') '15eca0aa3e2cc2b9b4fbe0629f1dda87f329500fcdcd6ef546d163211266b3b3'