You can duplicate a trip by simply going on your active itineraries page, choose the itinerary or proposal you wish to duplicate, and then press the Copy button. A copy will appear in your active trips list.
Here is what will be copied from one trip to a new one:
Itinerary
Trip dates
Location
Here is what is NOT copied:
Expenses
Bookings/Reservations
Discussions/Questions
Itinerary item tags
Trip price
Invites