I was modding years ago so I'm not up to date, even forgot lots of things. Now I remember I used to dublicate lots of statics and other objects after I dragged and dropped into the rendering window to save time. Now somehow I remember people talking about issues with dublicating objects. Can anyone explain?

Sometimes when you copy/paste objects from another cell, they will not line up correctly with the Grid in your cell.
Keep in mind that if you copy/paste objects, they retain the Ownership and Scripts that they had on them.
Other than that, I have never encountered anything worth mentioning.