I normally have my Sheet Sets saved out to a VCL file - that way when you import them the first time they come in as you defined them. The simplest way to "Duplicate" is to rename the first imported Sheet Set to e.g. Corridor 1 and then to import the VCL again and it creates you an empty second sheet set to work with. If you don't rename it then the second sheet set gets imported into the first imported sheet set and the Sheets are all intermingled.
To create a VCL of the Sheet Set that you created - you need to save your Project (save as so that you create a second instance is best). Then Delete all of your Plan View Data, Surfaces, Corridors, Alignments etc. so that the project is empty. Delete any sheets below the Master Sheets in the Plan Set so you just have the Master Sheets remaining.
Once you have this as you need it, select the Plan Set and all of your Master Sheets in the Project Explorer and then Export to a VCL file - you should store the VCL in the Drafting Templates Folder so you can easily find them (get the path by running the Drafting Templates Command which opens up the Drafting templates Folder in Explorer - you can copy the path from the Header area of File Explorer and then use the path when Exporting the VCL.
Once you have the VCL - run Drafting Templates command, find your VCL file and then drag and drop it into the Project - remember if it already exists in the project to first rename your existing Plan Set to e.g. Corridor 1 and then the second import will create a second Plan Set that you can also rename if you want to bring in a third instance etc.
To Rename a Plan Set - select the Plan Set in Project Explorer and look at the properties pane and you can change the name there.
Hope this helps
Alan