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You have the right visa, so your paperwork is good. Any country or particular border agent may question you but your single-entry Schengen visa allows you to enter the Schengen area exactly once. Since Iceland, Germany and Greece are all part of the area, you will only enter at your first landing point. On your way back, flying from Iceland to Frankfurt and then to Athens keeps you within the Schengen area, so do not need more than a single entry visa.