You can't pay accidentally—your contactless card or payment-enabled mobile/wearable device must be within 2 inches of the terminal for the transaction to take place. And you won't be billed twice, even if you accidentally tap twice.
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Because no signature or PIN verification is typically required, contactless purchases are often limited to a maximum amount per transaction, known as a Cardholder Verification Limit (CVM limit). Limits vary between banks.