The first five recorded levels begin with familiar groups, but a few words do more than fill one category. Level 1 has Farm Animals (Cow, Horse, goat and pig) and Colors (Red, Blue, Green and Yellow). Those are straightforward groups to verify by checking every word under the heading.
Level 3: School Leads to School Subjects
In Level 3, the School group lists Book, Pen, Desk and School Subjects. School Subjects then becomes another category heading, with Math, Science, History and Art beneath it. Compare both groups; the second one is not an alternative spelling of the first.
Level 4: Clothes Leads to Shoes
Level 4 lists Hat, Shirt, Pants and Shoes under Clothes. A separate Shoes group contains Lace, Sole, Aglet and Heel. This is a useful example of the game's connected category structure: a word from one group becomes the subject of another.
Level 5: Pets Leads to Cat
The Pets category in Level 5 contains Dog, Cat, Fish and Bird. Cat then heads a group with Whiskers, Purr, Scratch and Meow. If you think a word has been duplicated by mistake, check whether it is being used once as a group member and again as a category heading.