You can either duplicate an existing forecast, or you can create a new forecast with predictions from an existing forecast.
Duplicate an existing forecast
Open the Forecasts tab.
Select the Forecasts list tab.
Select the P&L budget or 3-way forecast you'd like to duplicate.
Select the Actions button.
Select Duplicate.
Enter a name, start date and duration (if applicable).
Select Save.
Use existing predictions in a new P&L budget or 3-way forecast
Open the Create option.
Select a P&L budget or 3-way forecast.
Enter a name, start date and duration (if applicable).
In the Add predictions dropdown, select Copy predictions from an existing forecast.
Select the forecast to copy.
Select Create.