Can a subscription customer purchase a second subscription for the same product?

Yes, customers can purchase additional subscriptions for the same product either through the checkout or directly in the customer portal. Customers can then manage multiple subscriptions through their customer portals. 

Customers must purchase any additional subscriptions using the same payment method and email address they did during their initial purchase. Recharge will notify you if an existing customer uses a different payment method or email address, providing you an opportunity to rectify this with your customer. This prevents any user confusion as to which account to use to manage certain subscriptions.