Production sync was failing every minute with:
UNIQUE constraint failed: order_items.order_number, order_items.sku
GoMag occasionally returns the same SKU on multiple lines within one order
(configurable products, promo splits). The order_items PK is
(order_number, sku), so the raw batch insert violates UNIQUE and aborts
the entire sync — blocking partner-mismatch updates, address refresh,
and items repopulation for already-imported orders.
Added _dedup_items_by_sku() helper. Applied in save_orders_batch
(cancelled/already/skipped paths) and add_order_items (retry/sync import
paths). Keeps first price/vat/name, sums quantities on collision.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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