feat: Increase verbosity of duplicate column error message

crates/polars-lazy/src/physical_plan/executors/projection_utils.rs

@@ -284,7 +284,17 @@ pub(super) fn check_expand_literals(
                 all_equal_len = false;
             }
             let name = s.name();
-            polars_ensure!(names.insert(name), duplicate = name);
+
+            if !names.insert(name) {
+                let msg = format!(
+                    "the name: '{}' is duplicate\n\n\
+                    It's possible that multiple expressions are returning the same default column \
+                    name. If this is the case, try renaming the columns with \
+                    `.alias(\"new_name\")` to avoid duplicate column names.",
+                    name
+                );
+                return Err(PolarsError::Duplicate(msg.into()));
+            }
         }
     }
     // If all series are the same length it is ok. If not we can broadcast Series of length one.

crates/polars-plan/src/logical_plan/builder.rs

@@ -544,7 +544,10 @@ impl LogicalPlanBuilder {
 
             if !output_names.insert(field.name().clone()) {
                 let msg = format!(
-                    "The name: '{}' passed to `LazyFrame.with_columns` is duplicate",
+                    "the name: '{}' passed to `LazyFrame.with_columns` is duplicate\n\n\
+                    It's possible that multiple expressions are returning the same default column name. \
+                    If this is the case, try renaming the columns with `.alias(\"new_name\")` to avoid \
+                    duplicate column names.",
                     field.name()
                 );
                 return raise_err!(polars_err!(ComputeError: msg), &self.0, into);

py-polars/tests/unit/test_errors.py

@@ -464,7 +464,7 @@ def test_with_column_duplicates() -> None:
     df = pl.DataFrame({"a": [0, None, 2, 3, None], "b": [None, 1, 2, 3, None]})
     with pytest.raises(
         pl.ComputeError,
-        match=r"The name: 'same' passed to `LazyFrame.with_columns` is duplicate",
+        match=r"the name: 'same' passed to `LazyFrame.with_columns` is duplicate.*",
     ):
         assert df.with_columns([pl.all().alias("same")]).columns == ["a", "b", "same"]