Applescript - Add prefixes in file name to an array

Question

I have a little script I am working on and I am stuck on this feature.
If I have a folder full of files with prefixes like
sm_test.png
sm_test2.png
med_test.png
med_test2.png
lg_test.png
lg_test2.png
I would like to add each unique prefix to an array without any duplicates.
{"sm", "med", "lg"}

Answer 1

Thank you so much Niel! Since you helped me with my other post. I have one last questions and I swear I will leave you alone. Basically I am trying to combine the two.
I have the script creating folders based on the prefixs then I need to move the files to their folders and remove the prefix. It seems to work when there is only one prefix but breaks when there is more then one.
set the_array to {}
tell application "Finder"
          try
                    set theLocation to the selection as alias
          on error
                    set theLocation to (folder of the front window as alias)
          end try
          repeat with this_file in (get files of theLocation)
                    set the_name to name of this_file
                    set the_prefix to items 1 thru ((offset of "_" in the_name) - 1) of the_name as string
                    if the_array does not contain the_prefix then set the_array to the_array & the_prefix
          end repeat
          repeat with e in the_array
                    if not (exists folder e of theLocation) then make new folder at theLocation with properties {name:e}
          end repeat
          set this_list to every file of theLocation
          set AppleScript's text item delimiters to {"_"}
          repeat with i in this_list
                    repeat with ee in the_array
                              if (name of i) begins with ee then
  move i to folder ee of theLocation
                              end if
                    end repeat
                    set name of (the result) to text item 2 of (get name of (the result)) as string
          end repeat
end tell