diff --git a/src/flask/app.py b/src/flask/app.py
index d6f15e8c..98437cba 100644
--- a/src/flask/app.py
+++ b/src/flask/app.py
@@ -1048,6 +1048,8 @@ class Flask(Scaffold):
         self.url_map.add(rule)
         if view_func is not None:
             old_func = self.view_functions.get(endpoint)
+            if getattr(old_func, "_flask_sync_wrapper", False):
+                old_func = old_func.__wrapped__
             if old_func is not None and old_func != view_func:
                 raise AssertionError(
                     "View function mapping is overwriting an existing"
diff --git a/src/flask/helpers.py b/src/flask/helpers.py
index 1a96e744..6a6bbcf1 100644
--- a/src/flask/helpers.py
+++ b/src/flask/helpers.py
@@ -780,4 +780,5 @@ def run_async(func):
 
         return async_to_sync(inner)(*args, **kwargs)
 
+    outer._flask_sync_wrapper = True
     return outer
diff --git a/tests/test_async.py b/tests/test_async.py
index 5893ff69..8c096f69 100644
--- a/tests/test_async.py
+++ b/tests/test_async.py
@@ -24,6 +24,7 @@ def _async_app():
     app = Flask(__name__)
 
     @app.route("/", methods=["GET", "POST"])
+    @app.route("/home", methods=["GET", "POST"])
     async def index():
         await asyncio.sleep(0)
         return request.method
@@ -57,7 +58,7 @@ def _async_app():
 
 
 @pytest.mark.skipif(sys.version_info < (3, 7), reason="requires Python >= 3.7")
-@pytest.mark.parametrize("path", ["/", "/bp/"])
+@pytest.mark.parametrize("path", ["/", "/home", "/bp/"])
 def test_async_route(path, async_app):
     test_client = async_app.test_client()
     response = test_client.get(path)