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A lot of lessons in the category "Rook and Other Endgames" are duplicates. That probably explains why there is 250 puzzles in that category, when it's probably around 50 being duplicated 5 time each.
For example the following are the same puzzle:
You can also see it for other puzzles if you start at the beggining of the category and you skip the lessons quickly, you'll notice a lot of puzzles are repeated.
Hey Silex777; I agree, some lessons are duplicated for retention purposeses; but not all are. In some problems, they want you to play white. Then the same position comes up later, and they want you to play black