app.post('/spankpay/callback', async (req, res) => {
    const sig = req.headers['x-spankpay-signature']
    // ... validate signature ...

    try {
        const firstUse = await redis.set(`spankpay-webhook:${sig}`, '1', {
            // The nx - Not Exists - flag ensures the key can only be set once
            nx: true,
            // The ex - EXpire - flag ensures the key will expire after an hour
            ex: 60 * 60,
        })
        if (!firstUse)
            return res.json({ received: true })

        // ... handle webhook ...
    } catch (e) {
        // If there is an error, clear the flag so that the webhook
        // will be processed on a subsequent request.
        // NOTE: your application must be careful not to leave the
        //       webhook in a partially processed state, otherwise
        //       there may be inconsistencies when it is retried.
        await redis.del(`spankpay-webhook:${sig}`)
        throw e
    }

    return res.json({ received: true })
})