IMPORTANT In order to duplicate an existing budget, you need to OWN the company (or have the proper manage rights), not "share" the company (or have the proper manage rights). Review that you are indeed the OWNER of the company first before duplicating an existing budget 

NOTE If you have the unlimited scenarios option in your subscription plan, you can create as many budget scenarios as you want. When you duplicate a budget, we copy-paste all the information and re-create a new budget which is an exact copy of the budget you were working on. Any changes to this new budget will not affect the original budget from which you made a copy. The only changes that affect all of your scenarios are the business settings and historical data (the past cannot be different from one budget to another - the past is the past!)

1- You can create an exact copy of a budget by pressing the Duplicate button in the TOP MENU.

2- Enter the new name of the new budget scenario and press Submit.

Related Articles (Please refer to the links below) 

- How to RENAME a budget

- How to ADD a blank budget

- How to SELECT a budget?