I called my brother and sister-in-law out on it. They have one son named John and another named Shane. Drives...me...nuts! So I pointed out to them that John and Shane are the same name. Shane is a form of John. I wasn't rude in the way that I said it, I just matter of factly pointed it out. They treated it like a joke, saying that my brother was so egotistical that he likes the idea of having not one but two sons named after himself. (His name is John and their son John is a Jr.) That was a joke, they didn't actually know that Shane is a form of John.
I don't care, I would have done the same thing as the OP. It drives me nuts nuts nuts when people use two different forms of the same freaking name for their children. And if they don't know *they should know*. And shouldn't do it. And if they're going to be dumb enough to do it, then somebody ought to let them know how *dumb* they are. Without actually using the word dumb, of course.