Duplicate arguments are reported as "???"

[danxmoran]

Stumbled across this while working on fixing #70. The SimpleArgParser reports any duplicate arguments it finds as "???", instead of the arg name (here's a test that asserts on the behavior).

It seems like the SimpleArgParser doesn't have any knowledge of the names attached to the values it consumes, so I'm wondering if the responsibility for checking duplicate names should be moved to some other enclosing parser.