azure-sdk-for-js: Unique Key Policy is not being enforced

• package: @azure/cosmos
• version: ^3.6.2
• Linux:
• nodejs
  • version: ^8.5.4
• Google Chrome
  • version/name: 88.0.4324.190 (Official Build) (64-bit)
• typescript
  • version: ^3.7.3

Describe the bug The container I have created is not following the unique key policy that I have set. I suspect this is due to the fact that both items are being persisted at the exact same time.

To Reproduce Steps to reproduce the behavior:

1. Insert two items that have the exact same _ts, subject and seq value.
2. Both items persist in the database.

Expected behavior I expect one or both of these documents to get rejected from persistence.

Screenshots

Additional context Here is the code I used to create the container and set the unique key policy:

await db.containers.createIfNotExists({
  id: eventstore,     
  partitionKey: { paths: ["/subject"] },    
  uniqueKeyPolicy: {     
     uniqueKeys: [   
         { paths: ["/seq"] }   
     ]    
  }   
})

I am also using a stored procedure to insert items into this database:

function bulkInsertItems(items) {
  var container = getContext().getCollection();
  var containerLink = container.getSelfLink();

  // Validate input
  if (!items) throw new Error("The array is undefined or null.");

  // End if there are no items
  if (items.length == 0) {
    getContext().getResponse().setBody(0);
  }

  items.map(item => {
    var isAccepted = container.createDocument(containerLink, item);

    // If the item fails to persist, throw an error and CosmosDB will handle rolling back
    if (!isAccepted) {
      throw new Error("Unable to create item");
    }
  })

  getContext().getResponse().setBody(items.length);
}