Among other things. Also, be cautious about importing from AD. Most companies do NOT keep a cleaned-up AD, and if you import from there, you often get both duplicates, and dead systems.
I'd start by looking at my AD and AD imports, and then look for the duplicate merge policy.
BTW, WHERE to find that and/or how to merge dups is different based on what version of NS you have. I'm pretty sure there's a KB on both though. Otherwise, at least let us know that, along with what products you own (asset does it differently).