Warn when setting used on the command line more than once

See emscripten-core#19938

emcc.py

@@ -684,6 +684,9 @@ def phase_parse_arguments(state):
   for s in settings_changes:
     key, value = s.split('=', 1)
     key, value = normalize_boolean_setting(key, value)
+    old_value = user_settings.get(key)
+    if old_value and old_value != value:
+      diagnostics.warning('unused-command-line-argument', f'-s{key} specified multiple times. Ignoring previous value (`{old_value}`)')
     user_settings[key] = value
 
   # STRICT is used when applying settings so it needs to be applied first before

test/test_other.py

@@ -7811,6 +7811,10 @@ def test_dash_s_bad_json_types(self):
     err = self.expect_fail([EMCC, test_file('hello_world.c'), '-sEXPORTED_FUNCTIONS=[{"a":1}]'])
     self.assertContained("list members in settings must be strings (not $<class 'dict'>)", err)
 
+  def test_dash_s_repeated(self):
+    err = self.expect_fail([EMCC, '-Werror', test_file('hello_world.c'), '-sEXPORTED_FUNCTIONS=foo', '-sEXPORTED_FUNCTIONS=bar'])
+    self.assertContained('emcc: error: -sEXPORTED_FUNCTIONS specified multiple times. Ignoring previous value (`foo`) [-Wunused-command-line-argument]', err)
+
   def test_zeroinit(self):
     create_file('src.c', r'''
 #include <stdio.h>