You would have to manually locate the original actors used for the unit you are COPYING from.
The difference I ASSUME there is, since there is that glitch with duplicate. is that. When you DUPLICATE is that is AUTO names the new unit for you as a COPY. and give it the ID of Copy as well. (You will also have to manually change any DUPLICATED checkboxed data's name and id to remove the word COPY and rename it to what you want. And that in istelf can break things.
But when you ADD Object you can set a new ID and name right from the start, so it is not in any way a Duplicate, even if you select a unit or data to copy from, as it populates the new unit with the new ID you choose, yet COPIES the field data from the other unit verbatum, even if you have named it differently.
Sometimes, I notice that Duplicating it will set the model for my duplicated unit to TRY and load a model named "asset/models/blahblah_copy.m3" instead of the orig models setting that doesnt include the suffix _copy. This new model of course, doesnt exist... unless I can make one. lol. Which I have no skill to do. So you usually have to manually change it back to the correct model you wanted.
So basically its trying to load a model based on your duplicated units ID and name, rather then using the orig units model.
THis glitch doesnt show up much for EVERY duplication. I tried to reproduce it on my own terms in regards to my explaination, but it wouldnt happen. It may be that theyve partially fixed this, or it only happens under certain circustances.