Ensure we don't try to create duplicate columns in dataset insert.

This may occur when the col name differs from the field name.

playhouse/dataset.py

@@ -242,6 +242,7 @@ def columns(self):
 
     def _migrate_new_columns(self, data):
         new_keys = set(data) - set(self.model_class._meta.fields)
+        new_keys -= set(self.model_class._meta.columns)
         if new_keys:
             operations = []
             for key in new_keys:

tests/dataset.py

@@ -464,6 +464,18 @@ def test_table_column_creation(self):
         self.assertEqual(table.columns, ['id', 'name'])
         self.assertEqual(list(table.all()), [{'id': 1, 'name': 'charlie'}])
 
+    def test_table_column_creation_field_col(self):
+        table = self.dataset['people']
+        table.insert(**{'First Name': 'charlie'})
+        self.assertEqual(table.columns, ['id', 'First_Name'])
+        self.assertEqual(list(table.all()), [{'id': 1, 'First_Name': 'charlie'}])
+
+        table.insert(**{'First Name': 'huey'})
+        self.assertEqual(table.columns, ['id', 'First_Name'])
+        self.assertEqual(list(table.all().order_by(table.model_class.id)), [
+            {'id': 1, 'First_Name': 'charlie'},
+            {'id': 2, 'First_Name': 'huey'}])
+
     def test_import_json(self):
         table = self.dataset['people']
         table.insert(name='charlie')