Freetagging vocabularies allow taxonomies to be built on the fly, whether that be by any contributor (à la wikiwiki), or just a reduced set of authors
For publicly writable taxonomies, it is probably best to leave vocabularies as they are created, to avoid interfering with users' choices. OTOH, for redacted/edited sites, consistency is typically wished amongst articles.
Freetagging vocabularies in 4.7 allow authors to choose from already defined terms, but
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due the the very “free” tagging mechanism, additional terms can be entered
The unavoidable consequence is that over time, several spellings will usually appear for terms that should be unique, as they would be under a controlled vocabulary
amend the terms edit form so that nodes using any term are made available to admins along with the term definition
allow automatic term changes from there: if a term if identified as a new spelling of another, allow the admin to automatically change the term in the disfavoured spelling for all nodes using it to the term with the favoured spelling, and delete the disfavoured spelling
maintain a marker of “latest revised term” within for each freetagging vocabulary, and allow the admin to skip directly to that node, and order on term creation date
Second level:
Scan vocabularies for potential duplicates, using various algoriths like soundex and porter stemmer, to suggest additional term fusions