feat(open-data-quality): detect duplicate rows in odq-csv

[aborruso]

Problem

Open data files often contain duplicate rows due to export errors or data entry mistakes. Currently not checked.

Proposed check

Phase 3 — Content, new check: phase3_duplicate_rows

• Detect exact duplicate rows (all columns match)
• Report: count of duplicates, percentage over total rows, example rows
• Severity: MAJOR (duplicate rows distort aggregations and statistics)

Inspiration

Article 5 Useful Python Scripts for Automated Data Quality Checks (KDnuggets, Feb 2026) — script 3 (duplicate record detector).

Implementation hint

DuckDB can detect exact duplicates efficiently:

SELECT COUNT(*) - COUNT(DISTINCT *) AS duplicate_count FROM read_csv_auto('data.csv');

Out of scope (for now)

Near-duplicate rows (fuzzy matching across all columns) — too expensive and domain-specific.