Handle case, when the same continuation was added multiple times

This may happen, because continuation is added outside of an outer
transaction, when AwaitingState is applied. If an outer transaction was failed,
when continuation has added, and retry was performed, the same continuation
may be added several times.

src/Hangfire.Core/ContinuationsSupportAttribute.cs

@@ -97,7 +97,19 @@ private void AddContinuation(ElectStateContext context, AwaitingState awaitingSt
             using (connection.AcquireDistributedJobLock(parentId, AddJobLockTimeout))
             {
                 var continuations = GetContinuations(connection, parentId);
-                continuations.Add(new Continuation { JobId = context.BackgroundJob.Id, Options = awaitingState.Options });
+
+                // Continuation may be already added. This may happen, when outer transaction
+                // was failed after adding a continuation last time, since the addition is
+                // performed outside of an outer transaction.
+                if (!continuations.Exists(x => x.JobId == context.BackgroundJob.Id))
+                {
+                    continuations.Add(new Continuation { JobId = context.BackgroundJob.Id, Options = awaitingState.Options });
+
+                    // Set continuation only after ensuring that parent job still
+                    // exists. Otherwise we could create add non-expiring (garbage)
+                    // parameter for the parent job.
+                    SetContinuations(connection, parentId, continuations);
+                }
 
                 var jobData = connection.GetJobData(parentId);
                 if (jobData == null)
@@ -111,11 +123,6 @@ private void AddContinuation(ElectStateContext context, AwaitingState awaitingSt
 
                 var currentState = connection.GetStateData(parentId);
 
-                // Set continuation only after ensuring that parent job still
-                // exists. Otherwise we could create add non-expiring (garbage)
-                // parameter for the parent job.
-                SetContinuations(connection, parentId, continuations);
-
                 if (currentState != null && _knownFinalStates.Contains(currentState.Name))
                 {
                     var startImmediately = !awaitingState.Options.HasFlag(JobContinuationOptions.OnlyOnSucceededState) ||
@@ -151,30 +158,37 @@ private void ExecuteContinuationsIfExist(ElectStateContext context)
                 // the state of a continuation, we should simply skip it.
                 if (currentState.Name != AwaitingState.StateName) continue;
 
-                if (continuation.Options.HasFlag(JobContinuationOptions.OnlyOnSucceededState) &&
-                    context.CandidateState.Name != SucceededState.StateName)
-                {
-                    nextStates.Add(continuation.JobId, new DeletedState { Reason = "Continuation condition was not met" });
-                    continue;
-                }
-
                 IState nextState;
 
-                try
+                if (continuation.Options.HasFlag(JobContinuationOptions.OnlyOnSucceededState) &&
+                    context.CandidateState.Name != SucceededState.StateName)
                 {
-                    nextState = JsonConvert.DeserializeObject<IState>(
-                        currentState.Data["NextState"],
-                        new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.Objects });
+                    nextState = new DeletedState { Reason = "Continuation condition was not met" };
                 }
-                catch (Exception ex)
+                else
                 {
-                    nextState = new FailedState(ex)
+                    try
+                    {
+                        nextState = JsonConvert.DeserializeObject<IState>(
+                            currentState.Data["NextState"],
+                            new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.Objects });
+                    }
+                    catch (Exception ex)
                     {
-                        Reason = "An error occurred while deserializing the continuation"
-                    };
+                        nextState = new FailedState(ex)
+                        {
+                            Reason = "An error occurred while deserializing the continuation"
+                        };
+                    }
                 }
 
-                nextStates.Add(continuation.JobId, nextState);
+                if (!nextStates.ContainsKey(continuation.JobId))
+                {
+                    // Duplicate continuations possible, when they were added before version 1.6.10.
+                    // Please see details in comments for the AddContinuation method near the line
+                    // with checking for existence (continuations.Exists).
+                    nextStates.Add(continuation.JobId, nextState);
+                }
             }
 
             foreach (var tuple in nextStates)