It is very easy to organize multiple proposals to live within one file on your Itineraries dashboard and this will allow you to keep a record of the proposals you had sent that client.
Simply type in the same file name on the General Info tab when creating new proposals for the same potential client and the proposal will be saved within the same file.
You can also change the Internal File Name by deleting what's there and entering a new File name or existing one to move the itinerary.
After you've typed in the name of a new file, click Create File Name and then press Enter.
When you move the file to Confirmed you will still be able to easily see the history of everything you had put together for that client, and the confirmed itinerary (or itineraries) will have the blue check mark.