BUG: check and raise on df.applymap with df having dupe columns

Author: y-p

Diff for: pandas/core/frame.py

@@ -4244,6 +4244,9 @@ def infer(x):
             if com.is_datetime64_dtype(x):
                 x = lib.map_infer(x, lib.Timestamp)
             return lib.map_infer(x, func)
+        #GH2786
+        if not self.columns.is_unique:
+            raise ValueError("applymap does not support dataframes having duplicate column labels")
         return self.apply(infer)
 
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