Hi Daesung!
On Stripe's end, we don't limit the number of times a given customer can subscribe to a given plan. The reasoning here is that this is often a business decision and there might be as many businesses that want to prevent this than businesses that want to let their customers do this.
If you subscribe the same customer to a given plan on two separate subscriptions, this customer will be charged twice on each billing cycle. You can also group those under one subscription and use the `quantity` parameter to change how much to charge in total. This is documented here:
https://stripe.com/docs/subscriptions/quantities
The right approach here is to block this in your own code so that no one can subscribe to a plan more than once. You would either keep track of the plan(s) a given customer has subscribed to or use our API to list all active subscriptions for that customer and confirm that plan has not been used yet.
Hope this clarifies the logic!
Best,
Remi