I don't recall seeing that error in anything after 2002, but prior to that I had it quite regularly ... in one application I redesigned (it was R14 if my mind serves me correctly) the user created around 140 layers with a command ... sometimes the layers existed sometimes they did not, but everytime the command was used there would be output anywhere from 0 to 140 lines telling the user the layer already existed, even though cmdecho was set to 0. Instead of checking for the existence of 140 layers, I finally decided it would be easier to simply create a blank drawing with all of the layer settings and insert it with the routine instead of create the layers programmatically, but then that was a unique situation ... I would typically use a lambda function to iterate through large numbers of layers, but alas, some people don't understand lambda and cringe whenever it is used (as was the project manager on this project)
Anyway, after some research, I don't think it is sonething that users would need to worry with if they are using a relatively new version of AutoCAD.