ros::init(argc, argv, "ns_test");
  std::cout << ros::this_node::getNamespace() << std::endl;
  ros::NodeHandle nh("~");
  std::cout << nh.getNamespace() << std::endl;

This is fine

$ rosrun ns_test ns_test __ns:=foo
/foo
/foo/ns_test

This ought to print the same

rosrun ns_test ns_test __ns:=/foo
//foo
/foo/ns_test

An additional / was prefixed to the provided namespace.

and this (which is somewhat ridiculous, but why not fix it also)

rosrun ns_test ns_test __ns:=//foo//
///foo//
/foo/ns_test

The namespace gets cleaned up getting into the NodeHandle, but the this_node returns exactly what was on the command line plus the additional forward slash.

Using the released kinetic version currently.