Hi @diegoversiani and @luiggiab,
The hook is being used by another plugin that sends an OTP message on woocommerce checkout. The plugin is called miniorange (https://wordpress.org/plugins/miniorange-otp-verification/). So this plugin adds a Verification Code field to the checkout form and this field is what gets duplicated during the subsequent ajax call.
So for now, we had to do what I would consider a messy fix by adding another action to the woocommerce_after_checkout_billing_form hook that removes the duplicated HTML elements on each ajax call.
We believe this problem occurs with all other plugins that use the woocommerce_after_checkout_billing_form to add HTML elements to the checkout form when fluid is installed because most other plugins will not be considering the multiple subsequent ajax calls.
Let us know what you decide to do and whether this is worth looking into.
