Before and after your customer books flights via your Duffel integration, you should make sure that you display the following message:
"Your flight booking will be bound by the fare rules and Conditions of Carriage of the airline(s). The information you provide will be sent to the airline so they can process your booking."
You should also make sure you comply with relevant data protection laws, based on the country where your business is based and/or where your customer is pased.
For example, if you're based in the European Union (EU) or European Economic Area (EEA), or you're serving customers based in the EU or EEA, you should make sure that you comply with General Data Protection Regulation (GDPR). This may include informing your customers that their personal information will be processed by Duffel.