It made sense, Mei Deng thought, recapping all that happened before him. It was the only other solution, that could prove Lin Xiaolong was the killer.
Why?
Because the one who stayed with the second captain the entire night during the feast had to have worn the same dress as to match Lin Xiaolong's.
In plain words, Lin Xiaolong carried out his plan thus; Firstly, he had a dummy dressed up to his eyes, so no one could tell the difference, and then had him stick to the second captain throughout the night, posing as him.
Secondly, he had two clothes ready, the one he used for the murder as the first. And the second, was one identical to what the double ganger had on. Which was why at the crime scene the footsteps were hid in such a haphazard manner. Because Lin Xiaolong needed time to change back into his own clothes.
