There is no explicit Duplicate button, but you can achieve the same result:

1. Open the template you want to copy in the Proposals View Builder (or load it into a proposal's Visual Proposal Builder from the Templates tab).
2. Make any changes you want in the copy.
3. In the left panel's Templates tab click Save Current as Template (this is different from the header's own Save button — in the Proposals View Builder that header button is itself labeled Save Template and updates the template you opened).
4. In the Save as Template pop-up, type a new Template Name, optionally edit the Category (a free-text field, defaults to Custom), pick a Type (Full Proposal or Section/Block Group), and click Save Template.

Because Save Current as Template never sends the id of the template you opened, it always creates a brand-new template record — leaving the original untouched and adding a second, independent one with the same starting content.