Tag style auto complete generate new term when not selected

Hello,
When using the tag style auto complete widget and we enter a term name that already exists in the vocabulary but we didn't select it from the list and wrote it manually exactly the same, the term is being duplicated.
This option is great when we creating a new terms on the fly but has a problem when we inserting the text manually.

The patch handle the problem by checking in the validation function of the widget that the term is exists, if he exists then the term id is added to the term id list.