diff --git a/avocado/utils/vmimage.py b/avocado/utils/vmimage.py
index d475ad7cb795339b0f48950c32944218e21637c5..238d0a42a4009390b3fd663b23eb726a4b0f8be4 100644
--- a/avocado/utils/vmimage.py
+++ b/avocado/utils/vmimage.py
@@ -351,10 +351,10 @@ def list_providers():
     """
     List the available Image Providers
     """
-    return [_ for _ in globals().itervalues()
-            if (_ != ImageProviderBase and
-                isinstance(_, type) and
-                issubclass(_, ImageProviderBase))]
+    return set(_ for _ in globals().itervalues()
+               if (_ != ImageProviderBase and
+                   isinstance(_, type) and
+                   issubclass(_, ImageProviderBase)))
 
 
 #: List of available providers classes
diff --git a/docs/source/utils/vmimage.rst b/docs/source/utils/vmimage.rst
index 3af886f9c33ef88a81bb8a60a1da68ed899baa57..83ca3ccfb937bc366d10720ac772b0c4c9984b64 100644
--- a/docs/source/utils/vmimage.rst
+++ b/docs/source/utils/vmimage.rst
@@ -97,7 +97,7 @@ with::
     class MyTest(Test):
 
         def setUp(self):
-            vmimage.IMAGE_PROVIDERS.append(MyProvider)
+            vmimage.IMAGE_PROVIDERS.add(MyProvider)
             image = vmimage.get('MyDistro')
             ...