At a brief glance it seems they ensure idempotency w.r.t. possible errors/network failures etc. not necessarily to multiple purchases done by the customer by mistake.

In that case I think some high velocity fraud filters should kick in (which they probably did). Indeed the purchase amount is very large, so you wouldn't really expect it to be a repeated high velocity buy. On the other hand Stripe can't really cover all possible edge cases all of the time.