Do a search in the help file for item
You'll want the one called Rpg::Item or something similar. It'll tell you what the element set is called (same for weapon, different for armor). Then just include a conditional for not having that before the item is pushed to the @data for that script
Putting a ! in front of the command is basically "is not"
Like so:
if whatever.include?(whatever2)
Will do it if it's there
if !whatever.include?(whatever2)
Will do it if it's not there