> ## Documentation Index
> Fetch the complete documentation index at: https://docs.algoward.xyz/llms.txt
> Use this file to discover all available pages before exploring further.
# Concurrent Double-Settle
> Simultaneous settlement races must never mint more than one on-chain transaction.
U3
Universal
Must-have
Free Shopping, Asset Theft
## What it proves
U2 tests *sequential* retries. U3 targets a different, harder bug class: a check-then-act
idempotency guard that's perfectly safe when requests arrive one at a time, but has a race window
under true concurrency — for example, two requests both passing a "has this been settled yet?"
check before either one records that it's claimed the settlement.
**Source:** USENIX Security '26 facilitator study — concurrency races enabling double-settlement
(Free Shopping / Asset Theft).
## How it works
Ward builds one signed payload, then fires **five simultaneous** `/settle` requests for it,
released from a single shared gate (`util/timing.ts`'s `dispatchConcurrently`) rather than relying
on incidental network jitter — so the race is reproducible run to run, not a matter of luck.
## Pass condition
The safety-critical assertion is the same as U2's — at most one **distinct** on-chain transaction
id may result, since identical signed bytes hash to the same txid deterministically:
```ts theme={null}
const distinctTxns = new Set(successes.map(o => o.value?.transaction).filter(Boolean));
const passed = distinctTxns.size <= 1;
```
Ward additionally records (as informative, non-blocking context) whether *exactly one* HTTP-level
success occurred — a facilitator with a correctly implemented concurrency lock should have exactly
one request "win" the claim, rather than every request racing algod directly and each echoing
success on the same eventual transaction. Both outcomes are reported, but only the distinct-txn
count gates pass/fail — see
[D12](/reference/decision-log#d12-u2u3s-retry-safety-assertion-is-no-distinct-double-settlement-not-literally-success-exactly-once).
When exactly one distinct transaction results, Ward also calls
`ChainAdapter.waitForConfirmation()` on it and records the real on-chain confirmation as
additional evidence.