Every product I've seen ship SMS one-time codes has the same bug on day one. It doesn't show up in staging, it shows up in the bill and in support tickets: users receive two codes, and the second one invalidates the first. Here's the sequence that causes it.
Source: [Dev.to](https://dev.to/smsrtdev7273/your-otp-resend-button-is-a-race-condition-and-its-charging-you-twice-46jp)