Thanks for your interest. It can be an error from both the ends - merchant as well as customer. In terms of customer, either your input credentials might go wrong which will result in payment failures. Double payment maybe due to the time delay taken for your first initial payment (might be an payment failure too) and you've made the same payment again. Whereas in this case, one of your payments can be revered easily by contacting your bank. There are certain websites with a poor maintenance which results in payment failures or double payments also this can be intentional too.