Your donor may report seeing the following error message after submitting their donation:
Error 11: A duplicate transaction has been submitted.
Authorize.net has protections in place to prevent duplicate transactions from being submitted. This error indicates that a transaction was submitted within one minute of a previous attempt with the exact same information.
This can sometimes happen if a donor's card was originally declined and the donor resubmits quickly without changing any information. It can sometimes happen if they need to correct the card security code. Some banks don't consider this a different item so the card information seems identical.
To resolve this issue instruct the donor to close their browser, wait 2-3 minutes, try the donation again, and make sure the correct card information is submitted.
More information on this error:
https://support.authorize.net/s/article/Error-11-A-duplicate-transaction-has-been-submitted