Let us seek above all to avoid a common error and be certain that our counterpoint is so varied that the same passage or harmonic progression is not repeated exactly, with the same consonances, rhythms and tones. For while such counterpoints if well written will be free from anything discordant or unpleasant to the ear, nevertheless to repeat them does not produce the pleasure that springs from variety. Besides, the composer would be thought by connoisseurs of the art to have a meagre store of ideas. For it would seem to them that be uses the same passage again because he cannot devise another counterpoint.
Gioseffo Zarlino, from his 1558 Le Istitutioni harmoniche, Part III : The Art of Counterpoint.
Good thing he never lived to see the appendix of musical borrowings from Winton Dean and John Merril Knapp’s books on Handel’s operas. He would have pulled his hair out.