There's currently no specific button to duplicate proposals. You can either create it from scratch or from a template.
The good news is because you can save a proposal as a template, it's a very simple, two-step process.
Step 1: Find the proposal you want to duplicate, click on it and hover over Options on the right. You should see this:
Step 2: Click 'Save as template' then give it a name.
Step 3: Create a new proposal and when prompted to select a template, choose the one you've just saved.
Your new proposal will now be exactly the same as the original one. If you don't need that template anymore, you can freely delete it without it affecting the content of your new proposal.