Less optimistic stripping of duplicate commands

Currently in the `gl.xml` file there is duplicated `glGetPointerv()` in GL 4.3 and also  `glProgramUniformMatrix4fvEXT()` in `EXT_separate_shader_objects` ES extension. It's thus needed to remove duplicates also inside the subset itself (at least until the XML file is fixed).

Maybe this isn't exactly pythonic way to do it, but nothing better came to my mind :-)