What is the standard way for the ATC to ask a pilot to repeat him/herself? There is a lot of information on how the pilot should ask the ATC to repeat a statement, but is there a standard way for the ATC to ask the pilot to repeat a readback, or any new information that the pilot provides?
Yes, and it is exactly the same: Say again.


  Station calling Georgetown Ground say again your callsign
  
  Lufthansa 123 say again speed
  
  Scandinavian 123 say again all after 'good morning'


Or simply


  United 345 say again