My GP said the same thing, that this is the first time she's heard of this - but the ob/gyn who is the one who actually does the removal surgery says it's very common. I don't know how common - I'll ask her at my follow-up visit 6 weeks post-surgery.
I asked her about damage, and she said no one can say for sure, but probably none. Even if there's scarring, it's just on two very small spots on the uterus, so as long as a fertilized egg doesn't attach at that exact spot it wouldn't cause a problem. Like, there would be less scar tissue than in someone who had previously had a cesarean section. Of course, that's in theory - I don't know what will happen when I actually try to get pregnant. (But since I've never been pregnant before, even if I had trouble conceiving - who could say whether it was the Mirena or just me? In other words, I guess we'll never know whether there's any damage.)