Treating MTN's 409 as a failure is what double-charges the customer

Follow the retry path once. The HTTP client gives up after thirty seconds, the queue marks the job failed, and Laravel replays it ^[5]. The replay carries the same X-Reference-Id, so MTN declines to charge again and answers 409 Conflict with a body of RESOURCE_ALREADY_EXIST ^[7]. A driver that treats anything outside 2xx as a failure throws there, the caller concludes nothing was charged, and mints a new reference ^[8]. That new reference is a new payment request, the customer gets a second prompt, and if he approves it he pays twice ^[8]. The developer's write-up on dev.to says his own driver did exactly this, and that the key worked as specified: the double debit came out of reading the response ^[8].

So the status to write on a 409 is Pending ^[10]. A 409 tells you the first request was accepted, not what it reached, and the response does not carry that state ^[9]. The caller polls from there ^[10].

MTN also constrains the key itself. X-Reference-Id must be a UUID, and an order number is refused without an explanation ^[6]. That means the key is not derivable from the order, so it has to be persisted before the first attempt and read back on the replay ^[2]. A retry has to reuse the same UUID; generating a fresh one turns it into a new request. Learning the UUID rule from an unexplained rejection is a debugging experience you get to have exactly once.

Wave has no idempotency header at all, and a repeated POST to /v1/checkout/sessions simply creates a second checkout session ^[11]. The only handle is client_reference, capped at 255 characters and echoed back on the webhook ^[12]. It becomes usable because the read side supports lookup by it: Wave session ids carry a cos- prefix, and the driver routes anything without that prefix to /v1/checkout/sessions/search?client_reference= ^[13]. The second session will exist regardless; the only defense is finding it before the customer pays it, so the rule is to check before replaying ^[14].

On Orange Money the account goes as far as the shape of the flow, a web redirect returning a payment_url, with order_id as the only identifier you own ^[16]. Whether the Wave pattern of detect-then-decide transfers there is not established by this material. The incidence is likewise undocumented: three providers appear in this material, and no duplicate-charge rate is reported ^[3]. The BCEAO numbers around it are about connectivity, not correctness. At the end of June, 80 participants were connected and 74 institutions were still in live testing ^[3]; the source does not say whether the 74 sit inside the 80, and if they do, six were through (80 - 74 = 6) ^[1]. Senegal led the union with 20 authorised institutions on 2 April ^[4]. The claim that the deadline crunch is producing this class of bug is the author's, and he is explicit that it shows up only in production ^[2][15].

What transfers is narrow and cheap to adopt. If your provider set includes Wave, that 255-character client_reference is your only reconciliation handle, and it has to be populated on the first call, not discovered on the retry ^[12].