Detailed error message on hook registration conflict

Author: vitek

src/pluggy/_hooks.py

@@ -337,7 +337,11 @@ def set_specification(
         specmodule_or_class: _Namespace,
         spec_opts: "_HookSpecOpts",
     ) -> None:
-        assert not self.has_spec()
+        if self.has_spec():
+            raise RuntimeError(
+                f"Hook {self.spec.name!r} is already registered "
+                f"within namespace {self.spec.namespace}"
+            )
         self.spec = HookSpec(specmodule_or_class, self.name, spec_opts)
         if spec_opts.get("historic"):
             self._call_history = []

testing/test_hookcaller.py

@@ -405,3 +405,19 @@ def hello(self, arg: int) -> int:
     pm.register(Plugin2())
     with pytest.raises(PluginValidationError):
         pm.check_pending()
+
+
+def test_hook_conflict(pm: PluginManager) -> None:
+    class Api1:
+        @hookspec
+        def conflict(self) -> None:
+            pass
+
+    class Api2:
+        @hookspec
+        def conflict(self) -> None:
+            pass
+
+    pm.add_hookspecs(Api1)
+    with pytest.raises(RuntimeError):
+        pm.add_hookspecs(Api2)