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
"Rook and Other Endgames" have A LOT of duplications
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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:
http://www.chess.com/chessmentor/view_lesson.html?id=2091&c_id=9
http://www.chess.com/chessmentor/view_lesson.html?id=2093&c_id=9
http://www.chess.com/chessmentor/view_lesson.html?id=2096&c_id=9
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.