This matches my thinking. The retrieval/lookup approach is exactly what I built - basically Option C with an observe-before-act pattern.

For APIs that support idempotency keys (Stripe, etc.), I use those. For ones that don't but have retrieval (most do), I check first before retrying.

The question I'm wrestling with: is the extra round-trip for the lookup worth it? Or should I just accept the edge cases where it duplicates?

What's your threshold for "critical enough to avoid duplicates"? Payments obviously yes, but what about notifications, reporting, analytics events?