diff --git a/IPython/nbconvert/nbconvertapp.py b/IPython/nbconvert/nbconvertapp.py
index 2b6f14d..c105728 100755
--- a/IPython/nbconvert/nbconvertapp.py
+++ b/IPython/nbconvert/nbconvertapp.py
@@ -278,6 +278,11 @@ class NbConvertApp(BaseIPythonApplication):
             basename = os.path.basename(notebook_filename)
             notebook_name = basename[:basename.rfind('.')]
             if self.output_base:
+                # strip duplicate extension from output_base, to avoid Basname.ext.ext
+                if getattr(exporter, 'file_extension', False):
+                    base, ext = os.path.splitext(self.output_base)
+                    if ext == '.' + exporter.file_extension:
+                        self.output_base = base
                 notebook_name = self.output_base
             resources = {}
             resources['profile_dir'] = self.profile_dir.location
@@ -293,11 +298,11 @@ class NbConvertApp(BaseIPythonApplication):
                       exc_info=True)
                 self.exit(1)
             else:
-                write_resultes = self.writer.write(output, resources, notebook_name=notebook_name)
+                write_results = self.writer.write(output, resources, notebook_name=notebook_name)
 
                 #Post-process if post processor has been defined.
                 if hasattr(self, 'postprocessor') and self.postprocessor:
-                    self.postprocessor(write_resultes)
+                    self.postprocessor(write_results)
                 conversion_success += 1
 
         # If nothing was converted successfully, help the user.
diff --git a/IPython/nbconvert/tests/test_nbconvertapp.py b/IPython/nbconvert/tests/test_nbconvertapp.py
index 24139a8..b9cbec3 100644
--- a/IPython/nbconvert/tests/test_nbconvertapp.py
+++ b/IPython/nbconvert/tests/test_nbconvertapp.py
@@ -127,6 +127,16 @@ class TestNbConvertApp(TestsBase):
             with open('notebook2.slides.html') as f:
                 assert '/reveal.css' in f.read()
 
+    def test_output_ext(self):
+        """test --output=outputfile[.ext]"""
+        with self.create_temp_cwd(['notebook1.ipynb']):
+            self.call('nbconvert --log-level 0 --to python '
+                      'notebook1.ipynb --output nb.py')
+            assert os.path.exists('nb.py')
+
+            self.call('nbconvert --log-level 0 --to python '
+                      'notebook1.ipynb --output nb2')
+            assert os.path.exists('nb2.py')
 
     def test_glob_explicit(self):
         """