Avoid duplicates when adding terms with rules

Hi,

i have a content type "work packages". Users can define particular work packages for other users. Those contents have only 1-word-titles, which represents some kind of types of those work packages. Now these titles should be also taxonomy terms for better categorization.

I'm trying to use rules for that. I created a vocabulary "package types". My rule set is the following:

1) Load a vocabulary ("package types")
2) Add a new term to vocabulary ([node:title])

It works fine until a user creates a content with a title/term that already exists. The rules creates another term with the same name (duplicate). How can I avoid this?