CLN: pandas.concat internal checks (pandas-dev#57996)

Author: mroeschke

pandas/core/reshape/concat.py

@@ -661,16 +661,13 @@ def _get_concat_axis(self) -> Index:
                 indexes, self.keys, self.levels, self.names
             )
 
-        self._maybe_check_integrity(concat_axis)
-
-        return concat_axis
-
-    def _maybe_check_integrity(self, concat_index: Index) -> None:
         if self.verify_integrity:
-            if not concat_index.is_unique:
-                overlap = concat_index[concat_index.duplicated()].unique()
+            if not concat_axis.is_unique:
+                overlap = concat_axis[concat_axis.duplicated()].unique()
                 raise ValueError(f"Indexes have overlapping values: {overlap}")
 
+        return concat_axis
+
 
 def _clean_keys_and_objs(
     objs: Iterable[Series | DataFrame] | Mapping[HashableT, Series | DataFrame],
@@ -768,6 +765,12 @@ def _concat_indexes(indexes) -> Index:
     return indexes[0].append(indexes[1:])
 
 
+def validate_unique_levels(levels: list[Index]) -> None:
+    for level in levels:
+        if not level.is_unique:
+            raise ValueError(f"Level values not unique: {level.tolist()}")
+
+
 def _make_concat_multiindex(indexes, keys, levels=None, names=None) -> MultiIndex:
     if (levels is None and isinstance(keys[0], tuple)) or (
         levels is not None and len(levels) > 1
@@ -780,6 +783,7 @@ def _make_concat_multiindex(indexes, keys, levels=None, names=None) -> MultiInde
             _, levels = factorize_from_iterables(zipped)
         else:
             levels = [ensure_index(x) for x in levels]
+            validate_unique_levels(levels)
     else:
         zipped = [keys]
         if names is None:
@@ -789,12 +793,9 @@ def _make_concat_multiindex(indexes, keys, levels=None, names=None) -> MultiInde
             levels = [ensure_index(keys).unique()]
         else:
             levels = [ensure_index(x) for x in levels]
+            validate_unique_levels(levels)
 
-    for level in levels:
-        if not level.is_unique:
-            raise ValueError(f"Level values not unique: {level.tolist()}")
-
-    if not all_indexes_same(indexes) or not all(level.is_unique for level in levels):
+    if not all_indexes_same(indexes):
         codes_list = []
 
         # things are potentially different sizes, so compute the exact codes