Some elements can be repeated in a page. That way you get an extra button and popup. To add these elements just click one of these elements in the panel on the left. You'll see an orange button with options to add delete or duplicate a popup. Some pages will have a maximum to the number of repeated elements you can add to the page. When you're inside a popup and have buttons or tabs, you will see the bar on the right showing a count of the tabs/popups inside the popup.
Having repeating elements can be good for when you want to be able to use multiple elements and such things as popups in one page to deliver more information or other content to the viewer.