If you ignore 3 sign-in codes (or click Resend code twice) in a row, you won’t be able to sign in until the most recent one expires.
After 10 minutes, your most recent code will expire and you can attempt to sign in again. Just click Back in the top-left corner and sign in as normal, using your mobile number or email address.
If you attempt to sign in on separate device while your device is locked you’ll see the following message: "A sign-in code is pending for a different device. Please wait a few minutes.”