For duplicating USB Flash drives I use ImageUSB, part of the free OSForensics toolbox, from PassMark Software / OSForensics.com
1. Load USB with tools / multiple OS
2. Create an image file from it with ImageUSB
3. Insert several USB drives into hub and then write back automagically!
Although originally designed for forensics work, its FREE and released under GPL. The other free tools may be of interest too
From the web page
http://www.osforensics.com/tools/write-usb-images.html
"ImageUSB is a free utility which lets you write an image concurrently to multiple USB Flash Drives. Capable of creating exact bit-level copies of USB Flash Drive (UFDs), ImageUSB is an extremely effective tool for the mass duplication of UFDs. ImageUSB can also be used to install OSFClone to a USB Drive for use with PassMark OSForensics"
"Warning: Due to the forensic nature of image duplication by ImageUSB, please ensure that you select UFDs with a storage size similar to the image you wish to duplicate. For example, if a 2GB image is copied to an 8GB USB Flash Drive, the drive will only be able to use two out of the eight gigabytes of storage space. In this scenario, users will need to reformat the UFD in order to access the rest of the storage space"
My usual disclaimer - I'm not affiliated with the software or site and incorrect use of this software may cause irrevocable data loss.