I’m having the same issue long term. No custom code, just paid shopify theme Prestige
Topic summary
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Core issue: Cart quantities intermittently duplicate (1 item becomes 2). Removing the “extra” unit can cascade, removing or adding quantities on other line items.
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Original context: Store using the Broadcast theme (bridetribes.com.au). Merchant receives ~5 customer emails per week reporting cart malfunctions; issue is not consistently reproducible.
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Diagnostics so far: One tester could not reproduce on the cart page. Another suggested potential JavaScript errors with an AJAX cart (cart updated via JavaScript without page reload) and asked for specifics on what “upsets the cart.” Merchant clarified the removal action affects other items in the cart.
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Additional reports: Similar, intermittent behavior on multiple themes—Prestige (paid), Dawn 2.4.0 (free), and Turbo 9.0—suggesting it may not be theme-specific. One user noted “Buy It Now” cleared the cart after adding items.
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Reproducibility: Merchants often cannot replicate; customers sometimes notice only post‑purchase.
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Status: No confirmed cause or fix shared. Open questions remain around specific JS/AJAX errors, conditions triggering duplication, and the “Buy It Now” interaction.