There should be no reason to install libc6-dbgsym if you already have libc6-dbg installed, but it's also true that they probably shouldn't overlap (the former should probably just be empty and depend on the latter).
There should be no reason to install libc6-dbgsym if you already have libc6-dbg installed, but it's also true that they probably shouldn't overlap (the former should probably just be empty and depend on the latter).