Drupal does not recognize when an author creates a term that is not saved as term but as synonym to another term in the vocabulary. It creates a new term and links the node to it instead of searching for this term as a synonym to another term and link to this other term.
So I always have to merge the terms by hand.
Taxonomy Manager should have a function that collects terms that are synonyms to other terms. It should merge these terms automatically.

Comments

Version:5.x-1.1» 6.x-2.x-dev

I second this request. Changing version to the D6 dev branch as D7 doesn't support synonyms anymore.