Fix wrapped view function comparison

Wrapped functions are not comparable, see
https://bugs.python.org/issue3564, therefore a marker is used to note
when the function has been sync wrapped to allow comparison with the
wrapped function instead.

This ensures that multiple route decorators work without raising
exceptions i.e.,

    @app.route("/")
    @app.route("/a")
    async def index():
        ...

works.

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"

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