lightning: don't wedge the listener on an already-credited preimage

Skip settlements whose preimage was already credited instead of retrying
until the process exits, and reject previously used preimages when they
are queued.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

1 files changed, 7 insertions, 0 deletions

diff --git a/routes/preimages.ts b/routes/preimages.ts
index 49673d17..5e7c484f 100644
--- a/routes/preimages.ts
+++ b/routes/preimages.ts
@@ -51,6 +51,13 @@ export default {
       for (let p of preimages) {
         p = p.toLowerCase();
         if (seen.has(p)) continue;
+        // A preimage that already settled (or backs a live invoice) can't be
+        // credited again — reject it rather than let a buyer pay for nothing.
+        if (
+          (await db.exists(`credited:${p}`)) ||
+          (await db.exists(`preimage:${sha256(p)}`))
+        )
+          fail("preimage already used");
         seen.add(p);
         fresh.push(p);
       }