What are some categories shown twice - like "cars" and "automotive"?

Because a significant number of people entered the field name in each of those ways during the review period. In order to prevent separation of similar data (for example, KudoZ terms), these identical (or almost identical) categories are tied together in the database. So it does not matter if you enter a question as an "automotive" or "car" question; it will be treated and shown in exactly the same way thereafter.

The best way to work with the detailed category structure is not to think about it too much!