The key, I think, in duplicating CDs using a computer burner is the speed. I've found the best success came with burning at the slowest burn rate that you can. I use K3B and burn at 4X, the slowest I can burn. The issue is the time it takes to burn multiple copies.
If you're going to do a lot of your own burning over a period of time you might have a look at the stand alone duplicators for your own use. They retail from about $250.00 and up and a number of burning bays can be added so multiple CDs can be burned at one time.