Index: Lib/ctypes/test/test_structures.py
===================================================================
--- Lib/ctypes/test/test_structures.py	(revision 59956)
+++ Lib/ctypes/test/test_structures.py	(working copy)
@@ -215,7 +215,16 @@
         # too long
         self.assertRaises(ValueError, Person, "1234567", 5)
 
+    def test_conflicting_initializers(self):
+        class POINT(Structure):
+            _fields_ = [("x", c_int), ("y", c_int)]
+        # conflicting positional and keyword args
+        self.assertRaises(TypeError, POINT, 2, 3, x=4)
+        self.assertRaises(TypeError, POINT, 2, 3, y=4)
 
+        # Should this raise TypeError instead?
+        self.assertRaises(ValueError, POINT, 2, 3, 4)
+
     def test_keyword_initializers(self):
         class POINT(Structure):
             _fields_ = [("x", c_int), ("y", c_int)]
Index: Modules/_ctypes/_ctypes.c
===================================================================
--- Modules/_ctypes/_ctypes.c	(revision 59956)
+++ Modules/_ctypes/_ctypes.c	(working copy)
@@ -3577,6 +3577,21 @@
 				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_TypeError,
+					     "duplicate values for field %s",
+					     field);
+				Py_DECREF(pair);
+				Py_DECREF(name);
+				Py_DECREF(fields);
+				return -1;
+			}
+
 			val = PyTuple_GET_ITEM(args, i);
 			if (-1 == PyObject_SetAttr(self, name, val)) {
 				Py_DECREF(pair);