在docker中配置ros-melodic，运行sudo apt-get update遇到的问题如下图所示：
找了很多说关于重新增加key的方法都不好用，最后运行了一下sudo apt-get upgrade，效果如下：

可以看出：

W: Target Packages (main/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list:28 and /etc/apt/sources.list.d/ros2.list:1
W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:28 and /etc/apt/sources.list.d/ros2.list:1
W: Target Packages (main/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list.d/ros-latest.list:1 and /etc/apt/sources.list.d/ros1-latest.list:1
W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list.d/ros-latest.list:1 and /etc/apt/sources.list.d/ros1-latest.list:1

然后把ros-latest.list和ros2.list两个文件都删掉，再运行sudo apt-get update，效果如下：
问题解决！