Those are multi-origin / split-fulfilment carts - when items ship from more than one location (multiple warehouses, a 3PL, or a subscription + one-time mix), Shopify requests a rate per fulfilment leg and adds them together, so a $7 rate can show as $14.
How to fix it:
Go to Settings → Checkout.
Turn on "Prevent multiplying shipping costs for carts identified as multi-origin." PDQ then apportions a single rate across the legs instead of summing them.
Leave it off if you only ever ship from one location.
💡 This can't be prevented 100% on express wallets (Apple Pay, Google Pay, Shop Pay) - they send only a partial address at rate time, so PDQ can't always identify the split cart. A known platform limitation, not a config error.