feat(swaps): check if rHash exists (#642)

This checks if the rHash for a swap request has been used for a previous
swap, and rejects the swap request if so. rHash values should not be
reused as we rely on them to uniquely identify swap they jeopardize the
atomicity of the swap.

Closes #547.

lib/swaps/Swaps.ts

@@ -304,7 +304,11 @@ class Swaps extends EventEmitter {
     // TODO: max cltv to limit routes
     // TODO: consider the time gap between taking the routes and using them.
     // TODO: multi route support (currently only 1)
-    // TODO: check to make sure we don't already have a deal for the requested payment hash
+
+    if (this.usedHashes.has(requestPacket.body!.rHash)) {
+      this.sendErrorToPeer(peer, requestPacket.body!.rHash, 'this rHash already exists', requestPacket.header.id);
+      return false;
+    }
     const requestBody = requestPacket.body!;
 
     const { quantity, price, isBuy } = orderToAccept;