There are two different ways to bring new models into an existing Navisworks model. One way is to append and one way is to merge. The first question I had the first time I used Navisworks was, what's the difference between appending a model and merging a model? You might be asking too. This actually has to do with the cleanliness of your models. If you have errors in your Revit or AutoCAD models, consisting of duplicate geometry, merging will remove these duplicates appending will not.