You can duplicate an existing campaign by clicking the Duplicate button, found in several locations in the portal:
My Campaigns - using list view:
The toolbar is located on the right side and is available for each individual campaign. You can find it next to the campaign's status by hovering with the mouse:
Click on the Duplicate icon to create another campaign.
Once a campaign has been duplicated, this window will appear:
You can chooseif you want to switch the campaign OS.
My Campaigns - using gallery view:
Hover with the mouse to use the quick toolbar found at the bottom of each box: