[ADD] product_unique_code.

Yeah well, it will also fail if the user already has, for whatever reason, another product with default code `orig_code-2`.
Finding a pseudo-random "free" default_code is imo overkill, the user should edit the code anyway, `-2` is just to show a default value. The same way I don't think users leave `(Copy)` in the product name when they duplicate it.

Maybe it's better to add a specific error that tells them to change the code before duplicating the product again?