X.U.EZConfig: Fix checkKeymap warning that all keybindings are duplicate

This reverts part of 42179b8625d83b2cd3c3a35da84de6f6c0dea6d6, which
effectively changed the duplicate check from >1 to >=1.

CHANGES.md

@@ -10,6 +10,10 @@
 
   * Fix build-with-cabal.sh when XDG_CONFIG_HOME is defined.
 
+  * `XMonad.Util.EZConfig`
+
+    - Fixed `checkKeymap` warning that all keybindings are duplicates.
+
 ### Other changes
 
 ## 0.18.0 (February 3, 20

XMonad/Util/EZConfig.hs

@@ -552,8 +552,8 @@ doKeymapCheck :: XConfig l -> [(String,a)] -> ([String], [String])
 doKeymapCheck conf km = (bad,dups)
   where ks = map ((readKeySequence conf &&& id) . fst) km
         bad = nub . map snd . filter (isNothing . fst) $ ks
-        dups = map (snd . NE.head)
-             . mapMaybe nonEmpty
+        dups = map (snd . NE.head . notEmpty)
+             . filter ((>1) . length)
              . groupBy ((==) `on` fst)
              . sortBy (comparing fst)
              . map (first fromJust)