What is a “dual” in chess problems, such as the Babson task? I’ve read up on Babson tasks, and they mention that all known tasks are not perfect because there are duals. I’m not sure what this means exactly. What precisely is a “dual” in chess problems?
A dual is an alternate solution, other than the intended one, after the first move of a problem or study. If it happens on the first move, the problem or study is called cooked instead.


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  Ideally, White should have only one move at each juncture that solves a problem. If White has an alternative at any stage other than the first move, this is a dual. A dual is not as serious a flaw as a cook, and in minor lines, duals may be permissible (opinions differ on this point). Some problems make a virtue out of dual avoidance – of two apparently equivalent white moves, only one works.


(source: Wikipedia)