diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 0185ca8241617..2d07e67654163 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2866,7 +2866,7 @@ def to_parquet(
         Returns
         -------
         bytes if no path argument is provided else None
-
+        
         See Also
         --------
         read_parquet : Read a parquet file.
@@ -8984,6 +8984,10 @@ def groupby(
 
         if level is None and by is None:
             raise TypeError("You have to supply one of 'by' and 'level'")
+        
+        if not as_index and isinstance(by, Series) and by.name in self.columns:
+            raise ValueError(f"cannot insert {by.name}, already exists")
+
 
         return DataFrameGroupBy(
             obj=self,