The easy answer to the question "How to repair book spine with duct tape?" is that you just never, and I mean it, never use duct tape and most of the other types of tape on books and documents of at least some value.
For all that the invention of scotch tape has made life easier, it has also made life much more difficult for those who now have to deal with the consequences of more than 150 years of tape application on paper and works of art.
Too often, repairs to comic books are done by people with good intentions without a better understanding or training in the field of conservation. Okay, there are cases of bad intentions and no training, as well.