StringIncrement() returning the same value on two different machines

We are using StackExchange.Redis (v2.0.601) to connect to Azure Redis. We are using StringIncrement() to increment the value of a key while inserting a record into another storage system, and then stop inserting when the Redis value exceeds a certain number. This is running on multiple machines with each service multi-threaded. This is a high load scenario where we are hitting StringIncrement() with hundreds or even thousands of requests per second, spread across multiple machines.

What I am seeing is that we are actually inserting more records than we expect. When I log the values being returned by StringIncrement(), ** I see that we are getting the same value returned on different machines**. Here is the code we are executing:

returnedValue = AzureCache(clusterIndex).StringIncrement(cacheKey, quantity)

The list below is shows a count of the value returned and the count of them.
|returned-value-after-increment|count-of-this-value-across-all-machines|
So, the value 208 was returned to 4 different threads. The value 3 was returned to 3 different threads.

  | 208 | 4 |  
  | 207 | 4 |  
  | 211 | 4 |  
  | 209 | 4 |  
  | 206 | 4 |  
  | 210 | 4 |  
  | 205 | 4 |  
  | 213 | 4 |  
  | 212 | 4 |  
  | 3 | 3 |  
  | 4 | 3 |  
  | 215 | 3 |  
  | 2 | 3

I thought that StringIncrement() is atomic and could never return the same value to two different threads/machines. Can anyone explain what is going on and how to fix this issue? Am I misunderstanding how this should work?

No source file or test is named. Start from the StringIncrement(cacheKey, quantity) call against Azure Redis and reproduce the high-load, multi-machine scenario described with StackExchange.Redis v2.0.601; done means explaining the repeated returned values and identifying a verified fix or limitation.