zabatonni

Automatically finds duplicate terms (with same name) in each taxonomy, then moves posts from duplicates to its original terms and deletes duplicate terms.
You can choose which duplicates you want to merge and delete.

Merge duplicate terms

Plugin for easy merging duplicate terms (tags, categories, ...)

changwoo

Over time, as you manage your website, you may accumulate a vast number of tags or non-hierarchical terms. This can lead to redundancy, with similar or even duplicate terms being created. Such overlapping terms can complicate your tagging scheme, prompting the need to consolidate multiple terms into a single, unified term.
While merging terms might seem straightforward at first glance, performing this task manually through the admin interface can quickly become cumbersome and time-consuming.
This plugin is designed to simplify the process, providing a more efficient and user-friendly solution for merging terms.

SWPMU Term Merger

A Simple Plugin for Merging Taxonomy Terms

Merge duplicate terms vs SWPMU Term Merger