Only if you go to your map's forest folder and duplicate correct items there. I think you can drag them to place after that, but never done such myself.
I'm not sure if they have unique identifiers the way TSS items do, BUT they do have their coordinates. So duplicating them would just make them appear in the exact same position and you would have to move each one individually anyway. Use static items instead, and you can duplicate them much easier within the editor. There is no benefit to using forest items anymore =)
Not sure if bug is fixed, but after adding enough many of them it starts to select randomly something else in addition to what you select. This has been so as long as I remember. It does not matter where you select, like for example selecting from list or from 3D view, still has been doing that for really long time. I can't remember if I tested with 0.15, I think that I did, at least some early 0.15 version. I can't remember where the issue started appearing, was it 4000 objects, less or more, but it had to be several thousands. With forest items at least 20 000 did work just fine.
