don't allow registering a class twice (fixes #218)
diff --git a/example/issues.cpp b/example/issues.cpp
index e164708..dfe20ff 100644
--- a/example/issues.cpp
+++ b/example/issues.cpp
@@ -131,4 +131,11 @@
 	    .def("f", &A::f);
 
 	 m2.def("call_f", call_f);
+
+    try {
+        py::class_<Placeholder>(m2, "Placeholder");
+        throw std::logic_error("Expected an exception!");
+    } catch (std::runtime_error &e) {
+        /* All good */
+    }
 }