Looks freaky the first time...
Entering a certificate number and finding it owned by someone else may seem strange a first. But it's happened to me before.
Could usually be that someone sells the coin on ebay and forgets to remove it from his registry set.
As far as the "research" is concerned -- NGC usually just sends an email to the owner and tells him/her that someone tried to enter the same certificate number into his registry set. It's happened to me before.