BUG: JSON serialization with orient split fails roundtrip with MultiIndex

doc/source/whatsnew/v2.0.0.rst

@@ -907,6 +907,7 @@ I/O
 - Fixed memory leak which stemmed from the initialization of the internal JSON module (:issue:`49222`)
 - Fixed issue where :func:`json_normalize` would incorrectly remove leading characters from column names that matched the ``sep`` argument (:issue:`49861`)
 - Bug in :meth:`DataFrame.to_json` where it would segfault when failing to encode a string (:issue:`50307`)
+- Bug in :meth:`DataFrame.to_json` where it would produce duplicate column names for orient=split (:issue:`50456`)
 
 Period
 ^^^^^^

pandas/io/json/_json.py

@@ -182,6 +182,20 @@ def to_json(
         indent=indent,
     ).write()
 
+    if orient == "split" and isinstance(obj, DataFrame):
+        if isinstance(obj.columns, MultiIndex):
+            lst = []
+            # backwards of multindex.fromArray
+            for i in range(len(obj.columns[0])):
+                sub = []
+                for j in range(len(obj.columns)):
+                    sub.append(obj.columns[j][i])
+                lst.append(sub)
+            newS = loads(s)
+            # fixes columns to original columns
+            newS["columns"] = lst
+            s = dumps(newS)
+
     if lines:
         s = convert_to_line_delimits(s)
 

pandas/tests/io/json/Untitled-1.ipynb

@@ -0,0 +1,19 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": []
+  }
+ ],
+ "metadata": {
+  "language_info": {
+   "name": "python"
+  },
+  "orig_nbformat": 4
+ },
+ "nbformat": 4,
+ "nbformat_minor": 2
+}

pandas/tests/io/json/test_pandas.py

@@ -1528,13 +1528,6 @@ def test_timedelta_as_label(self, date_format, key):
             ("index", "{\"('a', 'b')\":{\"('c', 'd')\":1}}"),
             ("columns", "{\"('c', 'd')\":{\"('a', 'b')\":1}}"),
             # TODO: the below have separate encoding procedures
-            pytest.param(
-                "split",
-                "",
-                marks=pytest.mark.xfail(
-                    reason="Produces JSON but not in a consistent manner"
-                ),
-            ),
             pytest.param(
                 "table",
                 "",

pp.py

@@ -0,0 +1,3 @@
+import pandas
+print(pandas.__version__)
+!pip install pytest