Duplicate option not detected when added

[pgerell]

According to the documentation, options_description::add should throw duplicate_variable_error.
This is not the case. Instead ambiguous_option is thrown when parsing the command line.
Ironically from within a function called find_nothrow

Many of the tests in https://github.com/boostorg/program_options/blob/develop/test/exception_test.cpp should throw duplicate_variabl_error