MAINT: Improve error message from sorting with duplicate key

Fixes #9082

2 files changed, 14 insertions, 5 deletions

diff --git a/numpy/core/_internal.py b/numpy/core/_internal.py
index f7f579c75..3902d2f7d 100644
--- a/numpy/core/_internal.py
+++ b/numpy/core/_internal.py
@@ -281,20 +281,26 @@ class _ctypes(object):
     _as_parameter_ = property(get_as_parameter, None, doc="_as parameter_")
 
 
-# Given a datatype and an order object
-#  return a new names tuple
-#  with the order indicated
 def _newnames(datatype, order):
+    """
+    Given a datatype and an order object, return a new names tuple, with the
+    order indicated
+    """
     oldnames = datatype.names
     nameslist = list(oldnames)
     if isinstance(order, str):
         order = [order]
+    seen = set()
     if isinstance(order, (list, tuple)):
         for name in order:
             try:
                 nameslist.remove(name)
             except ValueError:
-                raise ValueError("unknown field name: %s" % (name,))
+                if name in seen:
+                    raise ValueError("duplicate field name: %s" % (name,))
+                else:
+                    raise ValueError("unknown field name: %s" % (name,))
+            seen.add(name)
         return tuple(list(order) + nameslist)
     raise ValueError("unsupported order value: %s" % (order,))
 
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index 835d03528..e30cd09e7 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -30,7 +30,7 @@ from numpy.core.multiarray_tests import (
     )
 from numpy.testing import (
     TestCase, run_module_suite, assert_, assert_raises, assert_warns,
-    assert_equal, assert_almost_equal, assert_array_equal,
+    assert_equal, assert_almost_equal, assert_array_equal, assert_raises_regex,
     assert_array_almost_equal, assert_allclose, IS_PYPY, HAS_REFCOUNT,
     assert_array_less, runstring, dec, SkipTest, temppath, suppress_warnings
     )
@@ -1538,6 +1538,9 @@ class TestMethods(TestCase):
         assert_equal(r.word, np.array(['my', 'first', 'name']))
         assert_equal(r.number, np.array([3.1, 4.5, 6.2]))
 
+        assert_raises_regex(ValueError, 'duplicate',
+            lambda: r.sort(order=['id', 'id']))
+
         if sys.byteorder == 'little':
             strtype = '>i2'
         else: