feat: add column check for pairs

The root cause of the issue was that the column was renamed, and then
couldn't be found within the Polars data frame. This is a fairly naive
solution, and I'm open to ideas about how it might be improved.

lib/explorer/data_frame.ex

@@ -3609,6 +3609,7 @@ defmodule Explorer.DataFrame do
         df
 
       pairs ->
+        maybe_raise_column_duplicate(pairs)
         pairs_map = Map.new(pairs)
         old_dtypes = df.dtypes
 
@@ -3633,6 +3634,15 @@ defmodule Explorer.DataFrame do
     end
   end
 
+  defp maybe_raise_column_duplicate(pairs) when is_column_pairs(pairs) do
+    Enum.reduce(pairs, MapSet.new(), fn {col, _val}, seen ->
+      case col in seen do
+        true -> raise ArgumentError, "duplicate column name \"#{col}\" in rename"
+        false -> MapSet.put(seen, col)
+      end
+    end)
+  end
+
   defp check_new_names_length!(df, names) do
     width = n_columns(df)
     n_new_names = length(names)

test/explorer/data_frame_test.exs

@@ -2766,7 +2766,7 @@ defmodule Explorer.DataFrameTest do
     test "with keyword and a column that is duplicated" do
       df = DF.new(a: [1, 2, 3], b: ["a", "b", "c"])
 
-      assert_raise ArgumentError, ~r"duplicate column name \"g\"", fn ->
+      assert_raise ArgumentError, ~r"duplicate column name \"a\"", fn ->
         DF.rename(df, a: "first", a: "second")
       end
     end