After reading the OP (not the link) I'll admit I was horrified that anyone would consider keeping items they hadn't paid for.
Then, as I skimmed over the thread, I see that the persons in question tried to make things right and were gifted the items. This, I understand. Twenty odd years ago, my mom ordered a 24 kt gold baseball card and was sent two. Twice she sent the extra card back and twice it was sent back to her. She contacted the company but never was charged for the second item. At that point, there is little else a person can do.
However, if you don't at least TRY to make things right, you are a thief, plain and simple. Recently, my cousins wife was on facebook trying to sell an American Girl doll because the company had sent her two by mistake. I was, and still am, embarrassed that a "relative" would do such a thing.