diff --git a/cytoolz/itertoolz.pxd b/cytoolz/itertoolz.pxd
index 57e0609..8793a94 100644
--- a/cytoolz/itertoolz.pxd
+++ b/cytoolz/itertoolz.pxd
@@ -47,14 +47,28 @@ cdef class _unique_key:
     cdef object seen
 
 
+cdef class _nonunique_key:
+    cdef object key
+    cdef object iter_seq
+    cdef object seen
+
+
 cdef class _unique_identity:
     cdef object iter_seq
     cdef object seen
+    
+
+cdef class _nonunique_identity:
+    cdef object iter_seq
+    cdef object seen
 
 
 cpdef object unique(object seq, object key=*)
 
 
+cpdef object nonunique(object seq, object key=*)
+
+
 cpdef object isiterable(object x)
 
 
diff --git a/cytoolz/itertoolz.pyx b/cytoolz/itertoolz.pyx
index e138f71..17e924b 100644
--- a/cytoolz/itertoolz.pyx
+++ b/cytoolz/itertoolz.pyx
@@ -382,6 +382,10 @@ cdef class interleave:
         return val
 
 
+cdef class _iter_seq_key:
+    def __iter__(self):
+        return self
+
 cdef class _unique_key:
     def __cinit__(self, object seq, object key):
         self.iter_seq = iter(seq)
@@ -401,6 +405,25 @@ cdef class _unique_key:
         PySet_Add(self.seen, tag)
         return item
 
+cdef class _nonunique_key:
+    def __cinit__(self, object seq, object key):
+        self.iter_seq = iter(seq)
+        self.key = key
+        self.seen = set()
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        cdef object item, tag
+        item = next(self.iter_seq)
+        tag = self.key(item)
+        while not PySet_Contains(self.seen, tag):
+            PySet_Add(self.seen, tag)
+            item = next(self.iter_seq)
+            tag = self.key(item)
+        return item
+
 
 cdef class _unique_identity:
     def __cinit__(self, object seq):
@@ -419,6 +442,22 @@ cdef class _unique_identity:
         return item
 
 
+cdef class _nonunique_identity:
+    def __cinit__(self, object seq):
+        self.iter_seq = iter(seq)
+        self.seen = set()
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        cdef object item
+        item = next(self.iter_seq)
+        while not PySet_Contains(self.seen, item):
+            PySet_Add(self.seen, item)
+            item = next(self.iter_seq)
+        return item
+
 cpdef object unique(object seq, object key=None):
     """
     Return only unique elements of a sequence
@@ -432,6 +471,9 @@ cpdef object unique(object seq, object key=None):
 
     >>> tuple(unique(['cat', 'mouse', 'dog', 'hen'], key=len))
     ('cat', 'mouse')
+
+    See also:
+        nonunique
     """
     if key is None:
         return _unique_identity(seq)
@@ -439,6 +481,23 @@ cpdef object unique(object seq, object key=None):
         return _unique_key(seq, key)
 
 
+cpdef object nonunique(object seq, object 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
+    """
+    if key is None:
+        return _nonunique_identity(seq)
+    else:
+        return _nonunique_key(seq, key)
+
+
 cpdef object isiterable(object x):
     """
     Is x iterable?
@@ -473,11 +532,8 @@ cpdef object isdistinct(object seq):
     True
     """
     if iter(seq) is seq:
-        seen = set()
-        for item in seq:
-            if PySet_Contains(seen, item):
-                return False
-            seen.add(item)
+        for item in _nonunique_identity(seq):
+            return False
         return True
     else:
         return len(seq) == len(set(seq))
diff --git a/cytoolz/tests/test_itertoolz.py b/cytoolz/tests/test_itertoolz.py
index 1e77a26..69383bd 100644
--- a/cytoolz/tests/test_itertoolz.py
+++ b/cytoolz/tests/test_itertoolz.py
@@ -6,7 +6,7 @@
 from pickle import dumps, loads
 from cytoolz.itertoolz import (remove, groupby, merge_sorted,
                              concat, concatv, interleave, unique,
-                             isiterable, getter,
+                             nonunique, isiterable, getter,
                              mapcat, isdistinct, first, second,
                              nth, take, tail, drop, interpose, get,
                              rest, last, cons, frequencies,
@@ -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