diff --git a/src/library_fetch.js b/src/library_fetch.js
index 1467a90c5a279..88497f99bae70 100644
--- a/src/library_fetch.js
+++ b/src/library_fetch.js
@@ -13,9 +13,7 @@ var LibraryFetch = {
   $Fetch__postset: 'Fetch.staticInit();',
 #endif
   $Fetch: Fetch,
-  _emscripten_fetch_get_response_headers_length__sig: 'pi',
   _emscripten_fetch_get_response_headers_length: fetchGetResponseHeadersLength,
-  _emscripten_fetch_get_response_headers__sig: 'pipp',
   _emscripten_fetch_get_response_headers: fetchGetResponseHeaders,
   _emscripten_fetch_free: fetchFree,
 
@@ -26,7 +24,6 @@ var LibraryFetch = {
 #endif
   $fetchXHR: fetchXHR,
 
-  emscripten_start_fetch__sig: 'vp',
   emscripten_start_fetch: startFetch,
   emscripten_start_fetch__deps: [
     '$Fetch',
diff --git a/src/library_sigs.js b/src/library_sigs.js
index a503df85dcc8c..00e89727137a0 100644
--- a/src/library_sigs.js
+++ b/src/library_sigs.js
@@ -271,6 +271,9 @@ sigs = {
   _emscripten_dbg__sig: 'vp',
   _emscripten_dlopen_js__sig: 'vpppp',
   _emscripten_err__sig: 'vp',
+  _emscripten_fetch_free__sig: 'vi',
+  _emscripten_fetch_get_response_headers__sig: 'pipp',
+  _emscripten_fetch_get_response_headers_length__sig: 'pi',
   _emscripten_fs_load_embedded_files__sig: 'vp',
   _emscripten_get_now_is_monotonic__sig: 'i',
   _emscripten_get_progname__sig: 'vpi',
@@ -601,6 +604,7 @@ sigs = {
   emscripten_sleep__sig: 'vi',
   emscripten_stack_snapshot__sig: 'p',
   emscripten_stack_unwind_buffer__sig: 'ippi',
+  emscripten_start_fetch__sig: 'vp',
   emscripten_supports_offscreencanvas__sig: 'i',
   emscripten_throw_number__sig: 'vd',
   emscripten_throw_string__sig: 'vp',
diff --git a/src/utility.js b/src/utility.js
index 7738f041147a1..69979a7abbd74 100644
--- a/src/utility.js
+++ b/src/utility.js
@@ -145,8 +145,10 @@ function mergeInto(obj, other, options = null) {
       if (obj.hasOwnProperty(key)) {
         const oldsig = obj[key];
         const newsig = other[key];
-        if (oldsig != newsig) {
-          error(`Signature redefinition for: ${key}. (old=${oldsig} vs new=${newsig})`);
+        if (oldsig == newsig) {
+          warn(`signature redefinition for: ${key}. (old=${oldsig} vs new=${newsig})`);
+        } else {
+          error(`signature redefinition for: ${key}. (old=${oldsig} vs new=${newsig})`);
         }
       }
     }
diff --git a/system/lib/fetch/emscripten_fetch.c b/system/lib/fetch/emscripten_fetch.c
index cb86ebb6eee04..01edeb3a7cce2 100644
--- a/system/lib/fetch/emscripten_fetch.c
+++ b/system/lib/fetch/emscripten_fetch.c
@@ -17,6 +17,8 @@
 #include <emscripten/threading.h>
 #include <emscripten/console.h>
 
+#include "emscripten_internal.h"
+
 // From https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/readyState
 #define STATE_UNSENT           0 // Client has been created. open() not called yet.
 #define STATE_OPENED           1 // open() has been called.
@@ -31,12 +33,6 @@
 
 static void fetch_free(emscripten_fetch_t* fetch);
 
-// APIs defined in JS
-void emscripten_start_fetch(emscripten_fetch_t* fetch);
-size_t _emscripten_fetch_get_response_headers_length(int32_t fetchID);
-size_t _emscripten_fetch_get_response_headers(int32_t fetchID, char *dst, size_t dstSizeBytes);
-void _emscripten_fetch_free(unsigned int);
-
 typedef struct emscripten_fetch_queue {
   emscripten_fetch_t** queuedOperations;
   int numQueuedItems;
diff --git a/system/lib/libc/emscripten_internal.h b/system/lib/libc/emscripten_internal.h
index 819755001da8f..811934040fdbf 100644
--- a/system/lib/libc/emscripten_internal.h
+++ b/system/lib/libc/emscripten_internal.h
@@ -107,6 +107,13 @@ void _emscripten_throw_longjmp(void);
 
 void __handle_stack_overflow(void* addr);
 
+// Internal fetch API
+struct emscripten_fetch_t;
+void emscripten_start_fetch(struct emscripten_fetch_t* fetch);
+size_t _emscripten_fetch_get_response_headers_length(int32_t fetchID);
+size_t _emscripten_fetch_get_response_headers(int32_t fetchID, char *dst, size_t dstSizeBytes);
+void _emscripten_fetch_free(unsigned int);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/test/test_other.py b/test/test_other.py
index 621db8b286773..15e11150e155c 100644
--- a/test/test_other.py
+++ b/test/test_other.py
@@ -3849,7 +3849,7 @@ def test_js_internal_deps(self):
     err = self.run_process([EMCC, 'src.c', '--js-library', 'lib.js'], stderr=PIPE).stderr
     self.assertContained("warning: user library symbol 'jslibfunc' depends on internal symbol '$callRuntimeCallbacks'", err)
 
-  def test_js_lib_sig_mismatch(self):
+  def test_js_lib_sig_redefinition(self):
     create_file('lib.js', r'''
 mergeInto(LibraryManager.library, {
   jslibfunc__sig: 'ii',
@@ -3857,10 +3857,9 @@ def test_js_lib_sig_mismatch(self):
 });
 
 mergeInto(LibraryManager.library, {
-  jslibfunc__sig: 'dd',
+  jslibfunc__sig: 'ii',
   jslibfunc: function(x) {},
 });
-
 ''')
     create_file('src.c', r'''
 #include <stdio.h>
@@ -3869,8 +3868,18 @@ def test_js_lib_sig_mismatch(self):
   printf("c calling: %d\n", jslibfunc());
 }
 ''')
-    err = self.expect_fail([EMCC, 'src.c', '--js-library', 'lib.js'])
-    self.assertContained('lib.js: Signature redefinition for: jslibfunc__sig. (old=ii vs new=dd)', err)
+    err = self.run_process([EMCC, 'src.c', '--js-library', 'lib.js'], stderr=PIPE).stderr
+    self.assertContained('lib.js: signature redefinition for: jslibfunc__sig. (old=ii vs new=ii)', err)
+
+    # Add another redefinition, this time not matching
+    create_file('lib2.js', r'''
+mergeInto(LibraryManager.library, {
+  jslibfunc__sig: 'pp',
+  jslibfunc: function(x) {},
+});
+''')
+    err = self.expect_fail([EMCC, 'src.c', '--js-library', 'lib.js', '--js-library', 'lib2.js'])
+    self.assertContained('lib2.js: signature redefinition for: jslibfunc__sig. (old=ii vs new=pp)', err)
 
   def test_js_lib_invalid_deps(self):
     create_file('lib.js', r'''
diff --git a/tools/gen_sig_info.py b/tools/gen_sig_info.py
index 6467f7db4b883..a4d3a04eca79c 100755
--- a/tools/gen_sig_info.py
+++ b/tools/gen_sig_info.py
@@ -37,6 +37,7 @@
 #include <emscripten/html5.h>
 #include <emscripten/fiber.h>
 #include <emscripten/websocket.h>
+#include <emscripten/fetch.h>
 #include <emscripten/webaudio.h>
 #include <wasi/api.h>
 
@@ -228,12 +229,16 @@ def extract_sig_info(sig_info, extra_settings=None, extra_cflags=None):
     'STACK_OVERFLOW_CHECK': 1,
     'FULL_ES3': 1,
     'USE_SDL': 1,
+    'FETCH': 1,
     # Currently GLFW symbols have different sigs for the same symbol because the
     # signatures changed between v2 and v3, so for now we continue to maintain
     # them by hand.
     'USE_GLFW': 0,
-    'JS_LIBRARIES': ['src/library_websocket.js',
-                     'src/library_webaudio.js'],
+    'JS_LIBRARIES': [
+      'src/library_websocket.js',
+      'src/library_webaudio.js',
+      'src/library_fetch.js'
+    ],
     'SUPPORT_LONGJMP': 'emscripten'
   }
   if extra_settings:
diff --git a/tools/system_libs.py b/tools/system_libs.py
index 9287295df327f..7bebfb22b6bbc 100644
--- a/tools/system_libs.py
+++ b/tools/system_libs.py
@@ -1743,6 +1743,7 @@ def get_default_variation(cls, **kwargs):
 class libfetch(MTLibrary):
   name = 'libfetch'
   never_force = True
+  includes = ['system/lib/libc']
 
   def get_files(self):
     return [utils.path_from_root('system/lib/fetch/emscripten_fetch.c')]