The duplicate camera message is only shown if the Device ID matches for two or more entries in the Cameras list. In other words, you may have two entries for one of your cameras in the Cameras List instead of the two different cameras. The Device ID is unique for each webcam (even if they are the same Brand/Model).
The solution is to delete one of the duplicate entries and add the other camera from the Video Drop Down Menu.
Do you see both cameras in the Video Drop Down Menu?
