What is the clipboard?
Whenever you copy text, whether it be by highlighting the text, right clicking on it, and selecting copy, or by using a keyboard shortcut (such as ctrl+c), the text is copied to your computer's virtual clipboard. When Pasting the text, it copies the information that is on that same clipboard, then pastes it to the desired location.
When the text from your caption boxes have been copied to your clipboard, you can simply add a new caption box to your page, and then Paste that text in to the new caption box. This can be done by either clicking in the box and using the keyboard shortcut Ctrl + V (PC) or Cmd + V (Mac) by clicking on the Advanced button, and selecting Paste.
Note: When you paste the text into the new caption box, it will revert to the default text format. Any changes to the font/size/color will be lost. If the change is not what you want, you can always use the Undo button to change your Layout and captions back to the way they were formatted previously.