warn when registering same blueprint with same name

CHANGES.rst

@@ -33,7 +33,8 @@ Unreleased
 -   ``register_blueprint`` takes a ``name`` option to change the
     (pre-dotted) name the blueprint is registered with. This allows the
     same blueprint to be registered multiple times with unique names for
-    ``url_for``. :issue:`1091`
+    ``url_for``. Registering the same blueprint with the same name
+    multiple times is deprecated. :issue:`1091`
 
 
 Version 2.0.0

src/flask/blueprints.py

@@ -283,22 +283,34 @@ class Blueprint(Scaffold):
             name the blueprint is registered with. This allows the same
             blueprint to be registered multiple times with unique names
             for ``url_for``.
+
+        .. versionchanged:: 2.0.1
+            Registering the same blueprint with the same name multiple
+            times is deprecated and will become an error in Flask 2.1.
         """
-        first_registration = True
+        first_registration = not any(bp is self for bp in app.blueprints.values())
-
-        for blueprint in app.blueprints.values():
-            if blueprint is self:
-                first_registration = False
-
         name_prefix = options.get("name_prefix", "")
         self_name = options.get("name", self.name)
         name = f"{name_prefix}.{self_name}".lstrip(".")
 
-        if name in app.blueprints and app.blueprints[name] is not self:
+        if name in app.blueprints:
+            existing_at = f" '{name}'" if self_name != name else ""
+
+            if app.blueprints[name] is not self:
                 raise ValueError(
-                f"Blueprint name '{self.name}' "
+                    f"The name '{self_name}' is already registered for"
-                f"is already registered by {app.blueprints[self.name]}. "
+                    f" a different blueprint{existing_at}. Use 'name='"
-                "Blueprints must have unique names."
+                    " to provide a unique name."
+                )
+            else:
+                import warnings
+
+                warnings.warn(
+                    f"The name '{self_name}' is already registered for"
+                    f" this blueprint{existing_at}. Use 'name=' to"
+                    " provide a unique name. This will become an error"
+                    " in Flask 2.1.",
+                    stacklevel=4,
                 )
 
         app.blueprints[name] = self

tests/test_blueprints.py

@@ -140,7 +140,7 @@ def test_blueprint_url_defaults(app, client):
         return str(bar)
 
     app.register_blueprint(bp, url_prefix="/1", url_defaults={"bar": 23})
-    app.register_blueprint(bp, url_prefix="/2", url_defaults={"bar": 19})
+    app.register_blueprint(bp, name="test2", url_prefix="/2", url_defaults={"bar": 19})
 
     assert client.get("/1/foo").data == b"23/42"
     assert client.get("/2/foo").data == b"19/42"
@@ -873,9 +873,13 @@ def test_unique_blueprint_names(app, client) -> None:
     bp2 = flask.Blueprint("bp", __name__)
 
     app.register_blueprint(bp)
-    app.register_blueprint(bp)  # same name, same object, no error
+
+    with pytest.warns(UserWarning):
+        app.register_blueprint(bp)  # same bp, same name, warning
+
+    app.register_blueprint(bp, name="again")  # same bp, different name, ok
 
     with pytest.raises(ValueError):
-        app.register_blueprint(bp2)  # same name, different object
+        app.register_blueprint(bp2)  # different bp, same name, error
 
-    app.register_blueprint(bp2, name="alt")  # different name
+    app.register_blueprint(bp2, name="alt")  # different bp, different name, ok