diff --git a/ChangeLog.md b/ChangeLog.md
index a875400210aa0..75f7d15036dfa 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -20,6 +20,9 @@ See docs/process.md for more on how version tagging works.
 
 3.1.56 (in development)
 -----------------------
+- emscripten will now generate an `unused-command-line-argument` warning if
+  a `-s` setting is specified more than once on the command line with
+  conflicting values.  In this case the first setting is ignored. (#21464)
 
 3.1.55 - 03/01/24
 -----------------
diff --git a/emcc.py b/emcc.py
index f551c64fa2cce..0b1a963707719 100644
--- a/emcc.py
+++ b/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
diff --git a/test/common.py b/test/common.py
index fc74c360ef741..78bbd5d591ab9 100644
--- a/test/common.py
+++ b/test/common.py
@@ -868,7 +868,7 @@ def setUp(self):
     # For historical reasons emcc compiles and links as C++ by default.
     # However we want to run our tests in a more strict manner.  We can
     # remove this if the issue above is ever fixed.
-    self.set_setting('NO_DEFAULT_TO_CXX')
+    self.set_setting('DEFAULT_TO_CXX', 0)
     self.ldflags = []
     # Increate stack trace limit to maximise usefulness of test failure reports
     self.node_args = ['--stack-trace-limit=50']
diff --git a/test/test_browser.py b/test/test_browser.py
index f7a5e043ec833..0fb88a655adfd 100644
--- a/test/test_browser.py
+++ b/test/test_browser.py
@@ -4865,7 +4865,7 @@ def test_pthread_hello_thread(self, opts, modularize):
     'modularize': (['-sMODULARIZE', '-sEXPORT_NAME=MyModule'],),
     'O3': (['-O3'],),
     'O3_modularize': (['-O3', '-sMODULARIZE', '-sEXPORT_NAME=MyModule'],),
-    'O3_modularize_MINIMAL_RUNTIME_2': (['-O3', '-sMODULARIZE', '-sEXPORT_NAME=MyModule', '-sMINIMAL_RUNTIME=2'],),
+    'O3_modularize_MINIMAL_RUNTIME_2': (['-O3', '-sMODULARIZE', '-sEXPORT_NAME=MyModule', '-sMINIMAL_RUNTIME=2', '-Wno-unused-command-line-argument'],),
   })
   def test_minimal_runtime_hello_thread(self, opts):
     self.btest_exit('pthread/hello_thread.c', args=['--closure=1', '-sMINIMAL_RUNTIME', '-pthread'] + opts)
diff --git a/test/test_core.py b/test/test_core.py
index 2622dd04f39f6..428dd4b33a98a 100644
--- a/test/test_core.py
+++ b/test/test_core.py
@@ -9157,8 +9157,9 @@ def test_pthread_create_embind_stack_check(self):
     # embind should work with stack overflow checks (see #12356)
     self.set_setting('STACK_OVERFLOW_CHECK', 2)
     self.set_setting('EXIT_RUNTIME')
+    self.set_setting('DEFAULT_TO_CXX')
     self.emcc_args += ['-lembind']
-    self.do_run_in_out_file_test('core/pthread/create.c', emcc_args=['-sDEFAULT_TO_CXX'])
+    self.do_run_in_out_file_test('core/pthread/create.c')
 
   @node_pthreads
   def test_pthread_exceptions(self):
diff --git a/test/test_other.py b/test/test_other.py
index 7b50126d222e7..01e4828d25360 100644
--- a/test/test_other.py
+++ b/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>
@@ -12017,21 +12021,23 @@ def test_default_to_cxx(self):
 
   @parameterized({
     '': ([],),
-    'minimal': (['-sMINIMAL_RUNTIME', '-sSUPPORT_ERRNO'],),
+    'minimal': (['-sMINIMAL_RUNTIME'],),
   })
   def test_support_errno(self, args):
     self.emcc_args += args + ['-sEXPORTED_FUNCTIONS=_main,___errno_location', '-Wno-deprecated']
 
-    self.do_other_test('test_support_errno.c')
-    size_default = os.path.getsize('test_support_errno.js')
+    self.do_other_test('test_support_errno.c', emcc_args=['-sSUPPORT_ERRNO'])
+    size_enabled = os.path.getsize('test_support_errno.js')
 
     # Run the same test again but with SUPPORT_ERRNO disabled.  This time we don't expect errno
     # to be set after the failing syscall.
-    self.emcc_args += ['-sSUPPORT_ERRNO=0']
-    self.do_other_test('test_support_errno.c', out_suffix='_disabled')
+    self.do_other_test('test_support_errno.c', emcc_args=['-sSUPPORT_ERRNO=0'], out_suffix='_disabled')
 
     # Verify the JS output was smaller
-    self.assertLess(os.path.getsize('test_support_errno.js'), size_default)
+    size_disabled = os.path.getsize('test_support_errno.js')
+    print(size_enabled)
+    print(size_disabled)
+    self.assertLess(size_disabled, size_enabled)
 
   def test_assembly(self):
     self.run_process([EMCC, '-c', test_file('other/test_asm.s'), '-o', 'foo.o'])
diff --git a/test/test_sockets.py b/test/test_sockets.py
index 67f3c16a05747..5ecc0d4a329a0 100644
--- a/test/test_sockets.py
+++ b/test/test_sockets.py
@@ -332,7 +332,7 @@ def test_nodejs_sockets_echo_subprotocol(self):
   })
   def test_websocket_send(self, args):
     with NodeJsWebSocketEchoServerProcess():
-      self.btest_exit('websocket/test_websocket_send.c', args=['-lwebsocket', '-sNO_EXIT_RUNTIME', '-sWEBSOCKET_DEBUG'] + args)
+      self.btest_exit('websocket/test_websocket_send.c', args=['-lwebsocket', '-sWEBSOCKET_DEBUG'] + args)
 
   # Test that native POSIX sockets API can be used by proxying calls to an intermediate WebSockets
   # -> POSIX sockets bridge server