DData: PNCounterDictionary throws ArgumentException during pruning

[Aaronontheweb]

Found another bug once I fixed the leader selection code with the Replicator:

[Node #2(second)]Cause: System.ArgumentException: An element with the same key but a different value already exists. Key: y
[Node #2(second)]   at System.Collections.Immutable.ImmutableDictionary`2.HashBucket.Add(TKey key, TValue value, IEqualityComparer`1 keyOnlyComparer, IEqualityComparer`1 valueComparer, KeyCollisionBehavior behavior, OperationResult& result)
[Node #2(second)]   at System.Collections.Immutable.ImmutableDictionary`2.AddRange(IEnumerable`1 items, MutationInput origin, KeyCollisionBehavior collisionBehavior)
[Node #2(second)]   at System.Collections.Immutable.ImmutableDictionary`2.AddRange(IEnumerable`1 pairs, Boolean avoidToHashMap)
[Node #2(second)]   at Akka.DistributedData.ORSet`1.Prune(UniqueAddress removedNode, UniqueAddress collapseInto) in C:\Repositories\olympus\akka.net\src\contrib\cluster\Akka.DistributedData\ORSet.cs:line 353
[Node #2(second)]   at Akka.DistributedData.ORDictionary`2.Prune(UniqueAddress removedNode, UniqueAddress collapseInto) in C:\Repositories\olympus\akka.net\src\contrib\cluster\Akka.DistributedData\ORDictionary.cs:line 308
[Node #2(second)]   at Akka.DistributedData.PNCounterDictionary`1.Prune(UniqueAddress removedNode, UniqueAddress collapseInto) in C:\Repositories\olympus\akka.net\src\contrib\cluster\Akka.DistributedData\PNCounterDictionary.cs:line 154
[Node #2(second)]   at Akka.DistributedData.Internal.DataEnvelope.Prune(UniqueAddress from, PruningPerformed pruningPerformed) in C:\Repositories\olympus\akka.net\src\contrib\cluster\Akka.DistributedData\Internal\Internal.cs:line 431
[Node #2(second)]   at Akka.DistributedData.Replicator.PerformRemovedNodePruning() in C:\Repositories\olympus\akka.net\src\contrib\cluster\Akka.DistributedData\Replicator.cs:line 1362
[Node #2(second)]   at Akka.DistributedData.Replicator.ReceiveRemovedNodePruningTick() in C:\Repositories\olympus\akka.net\src\contrib\cluster\Akka.DistributedData\Replicator.cs:line 1275
[Node #2(second)]   at lambda_method(Closure , Object , Action`1 , Action`1 , Action`1 , Action`1 , Action`1 , Action`1 , Action`1 , Action`1 , Action`1 , Action`1 , Action`1 , Action`1 , Action`1 , Action`1 , Object[] )
[Node #2(second)]   at Akka.Tools.MatchHandler.PartialHandlerArgumentsCapture`16.Handle(T value) in C:\Repositories\olympus\akka.net\src\core\Akka\Util\MatchHandler\PartialHandlerArgumentsCapture.cs:line 761
[Node #2(second)]   at Akka.Actor.ReceiveActor.ExecutePartialMessageHandler(Object message, PartialAction`1 partialAction) in C:\Repositories\olympus\akka.net\src\core\Akka\Actor\ReceiveActor.cs:line 78
[Node #2(second)]   at Akka.Actor.UntypedActor.Receive(Object message) in C:\Repositories\olympus\akka.net\src\core\Akka\Actor\UntypedActor.cs:line 27
[Node #2(second)]   at Akka.Actor.ActorBase.AroundReceive(Receive receive, Object message) in C:\Repositories\olympus\akka.net\src\core\Akka\Actor\ActorBase.cs:line 158
[Node #2(second)]   at Akka.Actor.ActorCell.ReceiveMessage(Object message) in C:\Repositories\olympus\akka.net\src\core\Akka\Actor\ActorCell.DefaultMessages.cs:line 177
[Node #2(second)]   at Akka.Actor.ActorCell.Invoke(Envelope envelope) in C:\Repositories\olympus\akka.net\src\core\Akka\Actor\ActorCell.DefaultMessages.cs:line 83

Originally posted by @Aaronontheweb in #4196 (comment)