diff --git a/zarr/storage.py b/zarr/storage.py
index 2452cc5f1c..582ecfe7e2 100644
--- a/zarr/storage.py
+++ b/zarr/storage.py
@@ -7,6 +7,7 @@
 import zipfile
 import shutil
 import atexit
+import warnings
 
 
 import numpy as np
@@ -830,7 +831,9 @@ def __setitem__(self, key, value):
         if self.mode == 'r':
             err_read_only()
         value = ensure_bytes(value)
-        self.zf.writestr(key, value)
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", UserWarning)
+            self.zf.writestr(key, value)
 
     def __delitem__(self, key):
         raise NotImplementedError
diff --git a/zarr/tests/test_storage.py b/zarr/tests/test_storage.py
index bf03c96a21..24240068fa 100644
--- a/zarr/tests/test_storage.py
+++ b/zarr/tests/test_storage.py
@@ -8,6 +8,7 @@
 import array
 import shutil
 import os
+import warnings
 
 
 import numpy as np
@@ -645,6 +646,21 @@ def test_mode(self):
         with assert_raises(PermissionError):
             store['foo'] = b'bar'
 
+    def test_duplicate(self):
+        caught = False
+        with warnings.catch_warnings():
+            warnings.simplefilter("error", UserWarning)
+            try:
+                with ZipStore('example.zip', mode='w') as store:
+                    store['foo'] = b'bar'
+                    store['foo'] = b'baz'
+            except UserWarning:
+                caught = False
+            else:
+                caught = True
+
+        self.assertTrue(caught, "Failed to catch UserWarning.")
+
 
 def test_getsize():
     store = dict()