Fix all_completions output for both Jedi and non-Jedi cases

Closes  #11802.

IPython/core/completer.py

@@ -1139,10 +1139,10 b' class IPCompleter(Completer):'
         """
         Wrapper around the completion methods for the benefit of emacs.
         """
-        prefix = text[:text.rfind(".") + 1]
+        prefix = text.rpartition('.')[0]
         with provisionalcompleter():
-            return list(map(lambda c: prefix + c.text,
-                   self.completions(text, len(text))))
+            return ['.'.join([prefix, c.text]) if prefix and self.use_jedi else c.text
+                    for c in self.completions(text, len(text))]
 
         return self.complete(text)[1]
 

IPython/core/tests/test_completer.py

@@ -346,6 +346,24 b' class TestCompleter(unittest.TestCase):'
             )[1]
             nt.assert_equal(c, [escaped])
 
+    def test_all_completions_dups(self):
+        """
+        Make sure the output of `IPCompleter.all_completions` does not have
+        duplicated prefixes.
+        """
+        ip = get_ipython()
+        c = ip.Completer
+        ip.ex("class TestClass():\n\ta=1\n\ta1=2")
+        for jedi_status in [True, False]:
+            with provisionalcompleter():
+                ip.Completer.use_jedi = jedi_status
+                matches = c.all_completions("TestCl")
+                assert matches == ['TestClass'], jedi_status
+                matches = c.all_completions("TestClass.")
+                assert len(matches) > 2, jedi_status
+                matches = c.all_completions("TestClass.a")
+                assert matches == ['TestClass.a', 'TestClass.a1'], jedi_status
+
     def test_jedi(self):
         """
         A couple of issue we had with Jedi