How to remove duplicate terms in a vocabulary
The Term Merge module could help you.


  When using taxonomy for free tagging purposes, it's easy to end up with several terms having the same meaning. This may be due to spelling errors, or different users simply making up synonymous terms as they go.
  
  You, as an administrator, may then want to correct such errors or unify synonymous terms, thereby pruning the taxonomy to a more manageable set. This module allows you to merge multiple terms into one, while updating all fields referring to those terms to refer to the replacement term instead.