>Is it common practice to charge for extra copies to someone who has payed for the original development?, if not, why? >just as a good bussines practice or is it plain out unethical?
It depends on the terms than you agreed with your customer.
Many customers think than the app they buy, can install on any computer of his property.
Others, think than can install on any computer they want
Others think on resell copies or install on other computers and make money, and a high percent don't tell you.

Other thing is: I allways set an About form on where there are copyright notices and all legal stuff.

Have you a tracking system or security for the software you sell? yuo have records about your clients, copies sold, serial numbers, registration numbers, etc.?

Have you policies about your software, if don't, create them to protect your job.

Mauricio