Duplicate Content-Type header on Magento 1 API responses

Getting HTTP 500 errors with the message “duplicate header ‘Content-Type'” when using Magento 1 XmlRpc API? Here is how to fix them.

Beside the traditional way of using scripts to control Magento (for importers or fulfillment interfaces), you can also do so using Magento’s API. When using the XmlRpc API, you might get a surprise – depending on your web server configuration.