As far as I'm aware the cut-n-paste menu options only work within the Passage Editor.
I believe the simplest way to duplicate passages when using the Twine 2 application is to manual edit the related project HTML file externally within a Text Editor and use that applications cut-n-paste options.
WARNING: I would strongly suggest backing up your story project before doing any of the following!
You will need to be careful with doing this type of editing and make sure of the following:
1. Close down the Twine 2 application before externally editing the project HTML file.
2. Maintain the parent / child structure between the parent tw-storydata element and the child tw-passagedata elements, using a HTML or XML aware Text Editor will help with this.
3. Each tw-passagedata element must have the following properties:
a. an unique name String property.
b. an unique pid integer property.
c. (possible) a position co-ordinate value that doesn't over lap another passage.