I’ve read up on Babson tasks, and they mention that all known tasks are not perfect because they are duals. I’m not sure what this means exactly. Does it mean that the problem takes place in two parts of something? I would like an answer that goes in depth with good sources, not something that equates to “Duh! That’s the definition of a dual!”
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.
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.