How to Merge categories and tags?

I am using 2.7 and merged 5 blogs into one. The huge problem that has happened is that now I have a massive number of duplicate categories and tags. I’ve looked for a plugin that will merge the duplicates but nothing I’ve downloaded will actually do that in 2.7. I don’t want to sort the posts, I just need to either easily merge duplicate and similiar tags and categories or at least be able to sort the categories by title and have more than 25 on a page. Any advice would be so appreciated. I have about 800 tags/categories that are a total mess and I’m looking for an easy solution 🙂