It is a matter of some controversy. The worry was with the backdoor candidate. You get voted out on Friday, and you're back in on the list on Monday. That's how people refer to it in New Zealand. New Zealand and Germany do allow dual candidacy, and I think there is a good reason for it.
What you find is that without it people either want high on the list or safe seat. You want to have candidates contesting the ridings. If not, they still have a chance to be chosen on the list.
What you see in Germany and New Zealand is that 80%, or so, are dual candidacy. Yes, there is that perception—and it's discussed in the 2012 New Zealand report—but they concluded that dual candidacy was fine.