When G. F. Handel borrowed a melody of Gottlieb Muffat's for his Theodora he did so without any explicit attribution. On the other hand, one can say that he also didn't claim explicit credit for the borrowed material, only for the finished work. So I don't think it would be completely fair to conclude that Handel was trying to trick the public. Still I think that a modern writer who copies from the public domain in the same way that Handel copied from Muffat does well to try to make an explicit acknowledgement of source somewhere, sometime, rather than passing over the borrowing in silence as Handel did.

T.