Make an intermittent checkout failure reproducible

Support forwards three customer complaints from the last week: "I was charged twice for one order." Nobody in QA has been able to reproduce it. The developer has closed the ticket twice as "cannot reproduce."

What you know:

• Orders are created by POST /api/orders, which requires an Idempotency-Key header and returns 400 IDEMPOTENCY_KEY_REQUIRED without one.
• The intended contract: a repeated request with the same Idempotency-Key returns 200 with the already created order, not a new one.
• All three affected customers were on mobile networks. Two mentioned "the page hung and I tapped Place Order again."
• The orders table shows two rows with different ids, identical line items, identical totals, and timestamps 1.4s, 0.9s and 2.1s apart.

Your task: describe how you would make this reproducible on demand, what you would assert, and how you would prevent regression. An interviewer is testing whether you can convert a probabilistic field report into a deterministic test.