# ValueError: cannot reindex on an axis with duplicate labels

- **ID:** `python/pandas-cannot-reindex-duplicate`
- **Domain:** python
- **Category:** data_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

df.reindex() called on a DataFrame whose index (or columns) contains duplicate labels. Reindex requires unique labels to map old positions to new ones.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.x | active | — | — |

## Workarounds

1. **** (90% success)
   ```
   df = df.loc[~df.index.duplicated(keep='first')]; df.reindex(new_index)
   ```
2. **** (85% success)
   ```
   Aggregate first: df = df.groupby(level=0).sum(); df.reindex(new_index)
   ```

## Dead Ends

- **** — Same error, duplicates still present (90% fail)
- **** — Sorting doesn't dedupe (85% fail)