If you want to simply duplicate a page and make certain changes, you can just duplicate the page and use the page settings screen to move it to another publication. You can also click on the button Add Page and choose the Duplicate page from the menu.
You will also see the option Linked page. If you want to use the exact same page and also want to see changes in both publication when the page is changed, then this is the right option for you. It will show the same page in your publication but only the original can be changed.
Elements or groups can be imported in other pages or slides. Mouse over Options at the top right of the editor. Choosing the option Import Objects from the drop-down menu will open a popup. From here you can select which object you want to import from, by choosing the desired publication and page, also the scene if needed. It will then list all the elements in that page/scene, you can select the objects by clicking the hollow circle to the right of the object. Once you've decided, click the blue Import button at the bottom of the popup.
The objects you chose to import will then appear at the top of your layer listing, from here you are free to edit these elements as you please.