For nearly a decade, Drupal has offered powerful tagging and categorization features in the form of its Taxonomy module. There's a downside, of course: large pools of content or user-generated content often come with overlapping, inconsistent tags that can clutter your site's navigation and make search tools much less effective. The Term Merge module can help by providing tools to spot and combine duplicate taxonomy terms.
Term Merge adds a handful of additional screens to Drupal's standard Taxonomy forms. When browsing a given vocabulary, the Marge page allows administrators to choose any number of taxonomy terms, pick a destination term that they should be merged with, and control what other clean-up takss should be done once they're merged. Obviously, nodes tagged with the old terms can be tagged with the new merged one.
In addition, path redirects can be set up for the old term URLs. If the Synonyms module is installed, the old terms can also be saved as synonyms on the new merged one. If you're not sure where to start when hunting for unecessary terms to merge, the module can also search for duplicates: terms that frequently appear together, and can be merged without losing any information. It's a quick and easy way to cull duplicates from the site.
Term Merge solves a tangly problem, and does it well. If your site takes advantage of user-generated tags or has large pools of legacy tags and categories that need a clean-up, this module is a must-have.