routerrpc: de-duplicate htlc held by interceptor

This commit ensures htlcs currently held by the interceptor
Are not sent again. This prevents potential races in the user code that
may lead to loosing funds.

lnrpc/routerrpc/forward_interceptor.go

@@ -150,6 +150,11 @@ func (r *forwardInterceptor) holdAndForwardToClient(
 	htlc := forward.Packet()
 	inKey := htlc.IncomingCircuit
 
+	// ignore already held htlcs.
+	if _, ok := r.holdForwards[inKey]; ok {
+		return nil
+	}
+
 	// First hold the forward, then send to client.
 	r.holdForwards[inKey] = forward
 	interceptionRequest := &ForwardHtlcInterceptRequest{