is it in the game or can I now select a group of objects to duplicate.
It is Joël confired! So it is the truth! [haha]There will be a multi selection tool in Planet Coaster, but this feature is not yet available. It says: "coming soon". Look for it in the bottom right of the game UI.
Crtl click works in certain instances, but I think it only works within editing a building, so you can't do it for every piece out there. It only works in those cases, from what I have seen, where the 5 pieces of wall are joined as a building, you editing the building, then you can crtl click 3 pieces of the 5. You cannot select piece 6 which is another building.