As of October 12, 2016 we’ve made changes to how customer reference uniqueness is handled. Although the customer reference column was always required to be unique, previously the field was case-sensitive which allowed a reference to be duplicated by adjusting the casing, however this led to those subscriptions being invalid. The customer reference field will now be case-insensitive.
The changes made will affect all new customers going forward. We are in the process of updating all current customer references that will fail this new validation. If you have customers attached to active subscriptions with duplicate references, you will be receiving an email from us identifying the customers and providing guidance on how to fix them.