Index: Modules/_ctypes/_ctypes.c
===================================================================
--- Modules/_ctypes/_ctypes.c	(revision 59956)
+++ Modules/_ctypes/_ctypes.c	(working copy)
@@ -3577,6 +3577,20 @@
 				return IBUG("_fields_[i][0] failed");
 			}
 
+			if (kwds && PyDict_GetItem(kwds, name)) {
+				char *field = PyString_AsString(name);
+				if (field == NULL) {
+					PyErr_Clear();
+					field = "???";
+				}
+				PyErr_Format(PyExc_ValueError,
+					     "duplicate values for field %s",
+					     field);
+				Py_DECREF(pair);
+				Py_DECREF(fields);
+				return -1;
+			}
+
 			val = PyTuple_GET_ITEM(args, i);
 			if (-1 == PyObject_SetAttr(self, name, val)) {
 				Py_DECREF(pair);