The Duplicate Clips button is only available on the collection toolbar when the ClipHistory tab, the All Collections tab, or an AceText Collection tab is active.
When you click the Duplicate Clips button, a popup menu will appear below it. When you press Ctrl+D on the keyboard, the same menu appears to the right of the list of clips. The menu lists all AceText collections that you have opened. Select one of them from the menu to duplicate all selected clips and folders into that collection.
If the selected clips and folders are inside one or more folders, the same folder structure will be created in the collection you are duplicating the items into. If the folder structure already exists in the target collection, the items will be placed into the existing folders.
See Manage and Arrange Text to learn how to use AceText.