Prevent an argument with an already registered name to be added

except if it is -h/--help/-v/--version and they are default arguments.

For very complex programs with lots of options, this can help detecting
accidental redefinitions of already used argument names.

include/argparse/argparse.hpp

@@ -1545,6 +1545,7 @@ class ArgumentParser {
                           std::ostream &os = std::cout)
       : m_program_name(std::move(program_name)), m_version(std::move(version)),
         m_exit_on_default_arguments(exit_on_default_arguments),
+        m_default_arguments(add_args),
         m_parser_path(m_program_name) {
     if ((add_args & default_arguments::help) == default_arguments::help) {
       add_argument("-h", "--help")
@@ -2431,6 +2432,14 @@ class ArgumentParser {
 
   void index_argument(argument_it it) {
     for (const auto &name : std::as_const(it->m_names)) {
+      if (!((name == "-h" || name == "--help") &&
+            (m_default_arguments & default_arguments::help) == default_arguments::help) &&
+          !((name == "-v" || name == "--version") &&
+            (m_default_arguments & default_arguments::version) == default_arguments::version) &&
+          m_argument_map.find(name) != m_argument_map.end()) {
+        throw std::runtime_error("Argument with name " + name +
+                                 " already registered");
+      }
       m_argument_map.insert_or_assign(name, it);
     }
   }
@@ -2440,6 +2449,7 @@ class ArgumentParser {
   std::string m_description;
   std::string m_epilog;
   bool m_exit_on_default_arguments = true;
+  default_arguments m_default_arguments;
   std::string m_prefix_chars{"-"};
   std::string m_assign_chars{"="};
   bool m_is_parsed = false;

test/test_error_reporting.cpp

@@ -121,4 +121,16 @@ TEST_CASE("Detect unknown subcommand" * test_suite("error_reporting")) {
                            "Failed to parse 'logic', did you mean 'log'",
                            std::runtime_error);
   }
-}
+}
+
+TEST_CASE("Duplicate argument" * test_suite("error_reporting")) {
+  argparse::ArgumentParser program("program");
+  program.add_argument("-a");
+  program.add_argument("-h"); // redefining default arguments is allowed
+  program.add_argument("--help"); // redefining default arguments is allowed
+  program.add_argument("-v"); // redefining default arguments is allowed
+  program.add_argument("--version"); // redefining default arguments is allowed
+  REQUIRE_THROWS_WITH_AS(program.add_argument("-a"),
+                         "Argument with name -a already registered",
+                         std::runtime_error);
+}