If terms (admin OR user created) are duplicates, I don't see a way to delete them from within the Drupal browser interface.
This becomes a problem during, say, adding products to an ecommerce site that breaks their products up into categories. The person adding the product sees multiple duplicate categories (which are terms within a Vocabulary) and doesn't know which one to choose.
You can delete ALL terms in a lists by virtue of deleting a Vocabulary, but that's not an ideal solution.
If you go to the page where you can edit the term, the is also a option to delete the term.
The url to the page above is admin/content/taxonomy/edit/term/[tid]
, where tid is the term id for the term you want to delete. You can Click your way to it by:
list items
in the select vocabulary.