What logos appear on Duplo brick studs? Do all regular Duplo bricks have the word "Duplo" written on their studs? If not, what variations are there?
There really isn't a "on" for the Duplo studs.  All Duplo studs are hollow studs.  In the interior of the stud, depending on set and the year of the piece, the stud will read:


  DUPLO


or 


  LEGO