Made read_postgis raise ValueError if the geom_col is specified twice…

… (current error is cryptic) (#2849)

geopandas/io/sql.py

@@ -68,6 +68,12 @@ def _df_to_geodf(df, geom_col="geom", crs=None):
     if geom_col not in df:
         raise ValueError("Query missing geometry column '{}'".format(geom_col))
 
+    if df.columns.to_list().count(geom_col) > 1:
+        raise ValueError(
+            f"Duplicate geometry column '{geom_col}' detected in SQL query output. Only"
+            "one geometry column is allowed."
+        )
+
     geoms = df[geom_col].dropna()
 
     if not geoms.empty:

geopandas/io/tests/test_sql.py

@@ -734,3 +734,14 @@ def test_append_with_different_crs(self, engine_postgis, df_nybb):
         # Should raise error when appending
         with pytest.raises(ValueError, match="CRS of the target table"):
             write_postgis(df_nybb2, con=engine, name=table, if_exists="append")
+
+    def test_duplicate_geometry_column_fails(self, engine_postgis):
+        """
+        Tests that a ValueError is raised if an SQL query returns two geometry columns.
+        """
+        engine = engine_postgis
+
+        sql = "select ST_MakePoint(0, 0) as geom, ST_MakePoint(0, 0) as geom;"
+
+        with pytest.raises(ValueError):
+            read_postgis(sql, engine, geom_col="geom")