You can go into the settings for ES File Explorer and choose which directory is displayed when you run ES. (Menu->Settings->Home Directory)
If you want to fidn duplicate files on that SD card I would recommend mounting it to your PC and using something like Duplicate Cleaner
(or another application or utility appropriate to your OS).
Never ask before searching!
Current Device: AT&T LG Optimus G, stock, rooted JB
Retired Devices: At&T Galaxy Skyrocket, Dell Venue, Dell Streak, Moto Flipside, Moto Backflip, Samsung Eternity