Hey ,
Some time ago I contributed by creating unique SKU plugin which would prevent from adding 2 same SKUs to your webshop. As I believe that by sharing we will get the most out of it I hereby give you the code to first plugin from upcoming series of plugins.
source code is on github under link https://github.com/RafPe/nopcommerce-plugin-uniqueSku
Happy coding/sharing
Regards
Rafal