Avoid replacing file

I want to avoid the execution of shell script (i.e. mv command) if the file already exists in the destination folder. I am unable to do it. Please correct my mistake in the script below. The problem lies in

set dfolder to alias "7:V2:4:"
set destination to quoted form of POSIX path of dfolder
set ifolder to alias "8:Firefox Downloads:DownThemAll:"
set thepath to quoted form of (POSIX path of ifolder)

tell application "Finder"
	set dtafilenames to name of every file in dfolder
	set filenames to every file in folder ifolder
	repeat with afilename in filenames
		set y to count of (name of afilename as text)

		if name of afilename begins with "24" and (name of afilename ends with ".rm" or name of afilename ends with ".srt") and (y is 11) then
			set x to name of afilename
			set source to thepath & x
			if (not (exists x in dtafilenames)) then
				do shell script "mv " & source & space & destination
				end if

		end if
		end repeat
end tell


from the dictionary

exists (verb)Verify if an object exists (from Standard Suite)

a string or a custom list is no object

if x is not in  dtafilenames then

Using “mv -n” would do the trick I guess… Check “man mv” in Terminal for more info.

Thanks to both of you.