don't pair a lobby user twice in a row

to gracefully handle clients that could send `poolIn`
after having gotten a game.

for veloce/lichobile#1781

modules/pool/src/main/PoolActor.scala

@@ -19,6 +19,8 @@ final private class PoolActor(
 
   var members = Vector.empty[PoolMember]
 
+  private var lastPairedUserIds = Set.empty[User.ID]
+
   var nextWave: Cancellable = _
 
   implicit def ec = context.dispatcher
@@ -34,6 +36,9 @@ final private class PoolActor(
 
   def receive = {
 
+    case Join(joiner, _) if lastPairedUserIds(joiner.userId) =>
+    // don't pair someone twice in a row, it's probably a client error
+
     case Join(joiner, rageSit) =>
       members.find(joiner.is) match {
         case None =>
@@ -92,6 +97,8 @@ final private class PoolActor(
         monitor.ratingDiff(monId).record(p.ratingDiff)
       }
 
+      lastPairedUserIds = pairedMembers.view.map(_.userId).toSet
+
       scheduleWave()
 
     case Sris(sris) =>