This isn’t currently possible, but it would be quite simple to write some JS to do add another button to each layout and add in the duplicate functionality. If you wish to do this, p[lease dive into the pro/js/pro-input.js file and look at the acf.fields.flexible_content object and all it’s functions.
Otherwise, you will need to take the manually approach of adding a layout and then copy / paste the values across from one layout to another.
I can’t imagine why you would need to duplicate content in an email. Can you provide an example of when you would duplicate a layout including the values?
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