BUG: Join with list of dataframes that have MultiIndex now behaves as expected (pandas-dev#57676)

Author: Dacops

pandas/core/frame.py

@@ -10604,7 +10604,12 @@ def join(
             # "Iterable[Union[DataFrame, Series]]" due to the if statements
             frames = [cast("DataFrame | Series", self)] + list(other)
 
-            can_concat = all(df.index.is_unique for df in frames)
+            can_concat = True
+            for df in frames:
+                if isinstance(df.index, MultiIndex):
+                    can_concat = all(idx.is_unique for idx in df.index.levels)
+                elif isinstance(df.index, Index) and not df.index.has_duplicates:
+                    can_concat = False
 
             # join indexes only using concat
             if can_concat: