Symptoms
After executing an excel import you find that your categories are duplicated
Cause
Your excel import file is incorrectly formatted, or you have multiple locales running on your site.
Solution
Make sure that your excel import file is formatted correctly. The example file is in the ImportFileSamples directory.
If you have multiple locales running on your site, you cannot use the excel import. It will create a duplicate category for each additional locale that you have.