Allow identical (name, value) entry points

Author: ianthomas23

lib/matplotlib/backends/registry.py

@@ -168,8 +168,11 @@ def backward_compatible_entry_points(
     def _validate_and_store_entry_points(self, entries):
         # Validate and store entry points so that they can be used via matplotlib.use()
         # in the normal manner. Entry point names cannot be of module:// format, cannot
-        # shadow a built-in backend name, and cannot be duplicated.
-        for name, module in entries:
+        # shadow a built-in backend name, and there cannot be multiple entry points
+        # with the same name but different modules. Multiple entry points with the same
+        # name and value are permitted (it can sometimes happen outside of our control,
+        # see https://github.com/matplotlib/matplotlib/issues/28367).
+        for name, module in set(entries):
             name = name.lower()
             if name.startswith("module://"):
                 raise RuntimeError(

lib/matplotlib/tests/test_backend_registry.py

@@ -121,6 +121,17 @@ def test_entry_point_name_duplicate(clear_backend_registry):
             [('some_name', 'module1'), ('some_name', 'module2')])
 
 
+def test_entry_point_identical(clear_backend_registry):
+    # Issue https://github.com/matplotlib/matplotlib/issues/28367
+    # Multiple entry points with the same name and value (value is the module)
+    # are acceptable.
+    n = len(backend_registry._name_to_module)
+    backend_registry._validate_and_store_entry_points(
+        [('some_name', 'some.module'), ('some_name', 'some.module')])
+    assert len(backend_registry._name_to_module) == n+1
+    assert backend_registry._name_to_module['some_name'] == 'module://some.module'
+
+
 def test_entry_point_name_is_module(clear_backend_registry):
     with pytest.raises(RuntimeError):
         backend_registry._validate_and_store_entry_points(