C.,
I don't believe that it is anything for you to be overly concerned about. My son and daughter both had their teeth coming in from behind, and yes while it does look unappealing and doesn't seem normal, it's not unusual. My childrens orthodontist said that sometimes childrens teeth come in "faster" that the old teeth leaving and not uncommon, happens more often that people think. Maybe your child has a small mouth a jaw bone, like mine did, which was the cause of their teeth coming in double backed liked they did. I did take the time to have the smaller teeth removed and eventually my children both need braces, but today at the ages of 16 and 20 they both have very beautiful smiles. Both have small jaws and room inside for growth is limited, my son at 20 is looking to have his wisdom teeth removed due to no room. I would suggest taking your child to a dentist to have them checked for your own reassurance, but I'm sure you will find that there is nothing to worry about. Oh, and make sure that the dentist you choose does pediatric dental care, it makes a big difference for your children in the long run.