I'm Miles from the Social Care team at Shopify. Thanks for reaching out about this issue.
With the latest version of the POS app on iOS (6.16) some merchants have been having trouble with double scans happening. The double-scanning issue in POS should be fixed with the new update to the app, version 6.17. Please visit the App Store to update the app on your devices and that should resolve the issue.
If you're still seeing issues after updating the app, please let me know straight away so we can take a deeper look for you.
Apart from this issue, I hope Shopify POS is working well for your store. Are you currently using the Shopify POS Pro features? Remember, these are free to use until 31 October, after which it will cost $89 USD per month per location. If you don't need the Pro features, we do have Shopify POS Lite which will be free on all plans going forward. You can check out a full list of features here.