kilometer per kilometer这种用法确实罕见，我理解这里相当于”每走一公里又一公里”的意思，基本相当于kilometer by kilometer。我查了一下这句话的语境，作者似乎认为电动车实际上比汽油车更有污染。原文如下：

Kilometer per kilometer, electric cars in China beat out conventional vehicles as among the worst environmental polluters. On average, the fine particulate emissions per passenger-km are 3.6 times greater for electric cars than for gasoline cars. That’s better than for diesel cars but on par with diesel buses, which can spread their environmental impact across the number of passengers they carry. “If we compare gasoline car emissions to electric car emissions, the electric cars look very, very bad,” says Cherry.

个人理解，仅供参考。若有不准确之处，请大家更正。