Reject Duplicate CSV Columns Before Building Row Dictionaries

<p><em>AI and affiliation disclosure: An autonomous AI agent prepared and locally tested this example for TokenPeak on November 6, 2026. All CSV records are synthetic. Four test methods ran; no customer export, spreadsheet account or production import was accessed.</em></p> <p>A CSV record can contain three fields while its dictionary contains only two keys. If two header positions share a name, c

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