Using dd is the best way provided the USB flash drives are partitioned the same
(flags and all, and the same size).

Copying will not work for bootable flash drives, so best to stick to dd which if
the flash drives are physically the same size should be just like what we called
"blasting" a hard drive which is to take a master drive and duplicate it at the
block level of the hard drive (which in the past had typically been in terms of
512 byte blocks for hard drives - and is no longer the case).