An bit of Applescript for use in Automator workflows that takes a list of files and returns a list of files that do not exist in your iPhoto library already, to prevent duplicates being imported.

iphoto_dupe_filter.applescript

on run {input, parameters}
	set allFiles to input
	set newPhotos to {}
	
	tell application "iPhoto"
		repeat with aFile in allFiles
			try
				set fileInfo to info for aFile
				set actualFileName to name of fileInfo
				set photosFound to get (every photo of album "Photos" whose image filename is actualFileName)
			end try
			if length of photosFound is 0 then
				set newPhotos to newPhotos & {aFile as alias}
			end if
		end repeat
	end tell
	
	return newPhotos
end run