Issue #18988: The "Tab" key now works when a word is already autocompleted.
diff --git a/Lib/idlelib/AutoCompleteWindow.py b/Lib/idlelib/AutoCompleteWindow.py
index 298177f..27b0e56 100644
--- a/Lib/idlelib/AutoCompleteWindow.py
+++ b/Lib/idlelib/AutoCompleteWindow.py
@@ -157,13 +157,14 @@
         self.start = self.widget.get(self.startindex, "insert")
         if complete:
             completed = self._complete_string(self.start)
+            start = self.start
             self._change_start(completed)
             i = self._binary_search(completed)
             if self.completions[i] == completed and \
                (i == len(self.completions)-1 or
                 self.completions[i+1][:len(completed)] != completed):
                 # There is exactly one matching completion
-                return
+                return completed == start
         self.userwantswindow = userWantsWin
         self.lasttypedstart = self.start