System.ArgumentException: An item with the same key has already been added.

This error occasionally occurs after launching a web application. The reason is probably due to the concurrent processing of two HTTP requests, each calling enqueue.

```
System.ArgumentException: An item with the same key has already been added. Key: default
at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
at Hangfire.Azure.ServiceBusQueue.ServiceBusManager.CreateQueueClients()
at Hangfire.Azure.ServiceBusQueue.ServiceBusManager.GetSenderAsync(String queue)
at Hangfire.Azure.ServiceBusQueue.ServiceBusQueueJobQueue.DoEnqueueAsync(List`1 jobs)
```

Start at ServiceBusManager.CreateQueueClients and trace how concurrent calls from GetSenderAsync and ServiceBusQueueJobQueue.DoEnqueueAsync initialize queue clients. Reproduce two simultaneous enqueue requests during application startup, then verify that initialization no longer raises a duplicate-key ArgumentException.

Written by the indexing model from the issue text.