diff --git a/toolz/curried/__init__.py b/toolz/curried/__init__.py
index 356eddbd..30709bc9 100644
--- a/toolz/curried/__init__.py
+++ b/toolz/curried/__init__.py
@@ -77,6 +77,7 @@
 keymap = toolz.curry(toolz.keymap)
 map = toolz.curry(toolz.map)
 mapcat = toolz.curry(toolz.mapcat)
+nonunique = toolz.curry(toolz.nonunique)
 nth = toolz.curry(toolz.nth)
 partial = toolz.curry(toolz.partial)
 partition = toolz.curry(toolz.partition)
diff --git a/toolz/itertoolz.py b/toolz/itertoolz.py
index b8165162..1a3532f9 100644
--- a/toolz/itertoolz.py
+++ b/toolz/itertoolz.py
@@ -10,11 +10,12 @@
 
 
 __all__ = ('remove', 'accumulate', 'groupby', 'merge_sorted', 'interleave',
-           'unique', 'isiterable', 'isdistinct', 'take', 'drop', 'take_nth',
-           'first', 'second', 'nth', 'last', 'get', 'concat', 'concatv',
-           'mapcat', 'cons', 'interpose', 'frequencies', 'reduceby', 'iterate',
-           'sliding_window', 'partition', 'partition_all', 'count', 'pluck',
-           'join', 'tail', 'diff', 'topk', 'peek', 'peekn', 'random_sample')
+           'unique', 'nonunique', 'isiterable', 'isdistinct', 'take', 'drop',
+           'take_nth', 'first', 'second', 'nth', 'last', 'get', 'concat',
+           'concatv', 'mapcat', 'cons', 'interpose', 'frequencies', 'reduceby',
+           'iterate', 'sliding_window', 'partition', 'partition_all', 'count',
+           'pluck', 'join', 'tail', 'diff', 'topk', 'peek', 'peekn',
+           'random_sample')
 
 
 def remove(predicate, seq):
@@ -258,6 +259,9 @@ def unique(seq, key=None):
 
     >>> tuple(unique(['cat', 'mouse', 'dog', 'hen'], key=len))
     ('cat', 'mouse')
+
+    See also:
+        nonunique
     """
     seen = set()
     seen_add = seen.add
@@ -274,6 +278,34 @@ def unique(seq, key=None):
                 yield item
 
 
+def nonunique(seq, key=None):
+    """Return only the nonunique/duplicated elements of a sequence.
+
+    >>> tuple(nonunique((1, 2, 3, 1)))
+    (1,)
+    >>> tuple(nonunique((1, 2, 3)))
+    ()
+
+    See also:
+        unique
+    """
+    seen = set()
+    seen_add = seen.add
+    if key is None:
+        for item in seq:
+            if item in seen:
+                yield item
+            else:
+                seen_add(item)
+    else:
+        for item in seq:
+            val = key(item)
+            if val in seen:
+                yield item
+            else:
+                seen_add(val)
+
+
 def isiterable(x):
     """ Is x iterable?
 
@@ -305,12 +337,8 @@ def isdistinct(seq):
     True
     """
     if iter(seq) is seq:
-        seen = set()
-        seen_add = seen.add
-        for item in seq:
-            if item in seen:
-                return False
-            seen_add(item)
+        for item in nonunique(seq):
+            return False
         return True
     else:
         return len(seq) == len(set(seq))
diff --git a/toolz/tests/test_itertoolz.py b/toolz/tests/test_itertoolz.py
index 61618725..8262dc66 100644
--- a/toolz/tests/test_itertoolz.py
+++ b/toolz/tests/test_itertoolz.py
@@ -4,7 +4,7 @@
 from functools import partial
 from random import Random
 from pickle import dumps, loads
-from toolz.itertoolz import (remove, groupby, merge_sorted,
+from toolz.itertoolz import (nonunique, remove, groupby, merge_sorted,
                              concat, concatv, interleave, unique,
                              isiterable, getter,
                              mapcat, isdistinct, first, second,
@@ -105,6 +105,12 @@ def test_unique():
     assert tuple(unique((1, 2, 3), key=iseven)) == (1, 2)
 
 
+def test_nonunique():
+    assert tuple(nonunique((1, 2, 3))) == ()
+    assert tuple(nonunique((1, 2, 1, 3, 1))) == (1, 1)
+    assert tuple(nonunique((1, 2, 3, 4), key=iseven)) == (3, 4)
+    
+
 def test_isiterable():
     assert isiterable([1, 2, 3]) is True
     assert isiterable('abc') is True