Duplicate option not detected when added

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`