What causes an IAP receipt replayed to grant currency multiple times?

Your server validates a receipt as genuine but never records the transaction id, so a player resubmits the same valid receipt and is granted the currency again each time.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix an IAP Receipt Replayed to Grant Currency Multiple Times

Quick answer: Record each redeemed transaction id and reject any receipt whose transaction id was already granted, making redemption idempotent and single-use.

An attacker captures a real receipt and POSTs it to your grant endpoint ten times, getting ten payouts because validation passes every time. Tracking redeemed transaction ids fixes the replay.

How to fix it

1. Extract the transaction id

Parse the unique transaction id (and original transaction id for renewals) from the verified receipt rather than treating the whole receipt as the key.

2. Store redemptions

Persist each granted transaction id with a unique constraint; on redemption, insert-or-reject so a duplicate transaction id cannot grant a second time.

3. Validate against the store

Verify the receipt with Apple/Google server-side every time, but rely on the recorded transaction id, not just validity, to decide whether to grant.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

Ship the fix, watch the signature disappear from the next build. That's how you know it's really gone.